Hi KR7
I'd have to agree with Justiciar to a large extent. I'd be wary about the value of using a PC-based flight sim for
learning to fly. For me, this seems to be the salient point here, perhaps one that may have been
slightly missed by some contributors. If you already have flying experience and you wish to use flight sims to extend the fun then that is all well and good -- just as Justiciar said about instrument flying (a flight sim can be a good way to retain some level of currency on instrument flying). But I feel you're asking about the suitability of using MSFS (for example) as a learning tool and this is where things start to be subjected to personal opinions.
I'm aware that some FAA schools in the US actually give their students copies of MSFS when they start training. I'm aware that there are instructors who don't see the problem in using them. To counter this, I'd point out that these flying schools offer the software
in conjunction with actual flight training which is undertaken concurrently, not 'instead of' or that occurs subsequent to the flightsimming.
All I'm offering is a personal opinion, and my opinion is that it probably isn't desirable to actually
learn to fly using a computer.
mstram makes some valid points about the enjoyment of flight sims but they would appear to me to be from the standpoint of somebody who already knows how to fly, rather than someone who learned on a PC and transferred those skills to the real environment.
As I see it, it's all about the management of expectations. If you learn how to do something within a set environement, you form expectations of how certain things will occur. When you come to exercise those skills in a different environment and you find that things don't occur as you expect them, it leads to a high mental workload while you 'unlearn' what you think you know and seek to adapt to the new situation. This applies to any kind of training, not just aviation.
As an aviation-related aside, this doesn't just apply to PC flight sims. I was recently talking to a guy who has just become a first officer on the ERJ. We were talking about his base-training and he commented on how strange it was at first. He then explained that it all stemmed from the fact that, in certain phases of flight, the simulator flies nothing like the real aircraft does. So he had to make an allowance for certain expectations that he'd formed not being matched by reality. He had, of course, been warned about these 'gotchas' at the start of his training, but even a professional, multi-million dollar synthetic training device can occasionally be unrepresentative of the real thing.
So, to finish my rather rambling introduction, I would say that if you could get some real flying done first, before using MSFS, then do so. It is much easier to say "that's not how it is in real life" when using the sim than it is to say "whoa, that's not what I was expecting" when sitting in the real thing. Failing that, take up gliding. It's largely the same except without the need for engine management :P and normally a lot cheaper. But.... if you can't get real flying time before using the PC to learn and you're adamant you want to go down that route, then be mindful of the pitfalls.
Now... I am no flying instructor. So my advice to you would be to try and seek a qualified one who also does a fair bit of PC simming and ask them to explain the things you might need to be mindful of. I think that'd be a useful exercise. However, I will throw in a couple of extra thoughts as examples, but these are all based on my observational opinions and you should seek the thoughts of Mr. or Mrs. Qualified Instructor.
- Fly by reference to attitude, not instruments. A good way of doing this is to fly without the panel showing which, if I remember correctly, is done by pressing 'W' in MSFS (somebody will no doubt correct me if this is wrong). This will remove the panel and leave you with just the essential instruments along the bottom, and an unrestricted view of the outside. This still has its problems though, in that you then have an unrealistic level of visibility. One of the biggest problems with flight sims, in my opinion, is that you either see too much of the outside or not enough, at least when trying to fly by visual references. Flying visually is better done without the panel rather than with it, but without it you see 'too much' sometimes.
- A consequent problem is that in the climb, or any other nose-high attitude, you will still see very little out of the front of the PC aircraft. In reality you would have the benefit of peripheral vision to either side of the nose to help maintain attitude. I find that one lacks peripheral vision on a PC. This leads nicely to...
- ...landing! You will be have no problems learning to fly a stable approach on a flight sim. But I'd recommend that you don't ever believe that landing any plane in MSFS will mean you'll be able to do so in real life. It just isn't the same. Again, due in large part to the lack of peripheral vision, which is essential in gauging the flare and round-out. Also, in reality you will experience ground-rush, where the world begins to accelerate past you as you approach the ground, and this can serve as a useful visual cue. Maybe it's just me, but whenever I've had a go on a PC flight sim, that feeling has never been authentically recreated.
- Trimming. I'm sorry, but I have to disagree with mstram here. Yes, trimming in FS is a similar experience, if you know what the differences are, but in terms of learning to trim it is nothing like good enough. Allow me to explain the differences. Your PC joystick or yoke will have a centrepoint in the pitch axis. If you move the stick forward of the centrepoint, the aircraft will pitch down. If you move the stick aft of the centrepoint, the aicraft will pitch up. Simple. The problem is that the centrepoint is fixed. If you let go of the yoke, it will always spring back to the centrepoint. You can always move the stick forward-of-centre by, for example, three inches and always aft-of-centre by three inches. The neutral point stays the same.
It's not like that in an aircraft. That neutrality -- that centrepoint -- actually moves according to the aerodynamic forces on the pitching control surfaces. When you trim, you are trimming off the forces required to hold the column in its new neutral position. So that when you let go, it stays there.. if you've done it right! 
To use the same example as before, when trimmed for take off you might be able to move the yoke forward three inches and back three inches. However, once you've trimmed off the forces in the climb configuration, for example, you might find that you can move the yoke forward four inches, but only back two. The centrepoint has moved.
This is why I consider PC's and the real thing fundamentally different when it comes to trimming. The PC will not allow you to experience the control forces and, whatismore, it requires you to add or remove trim as you slowly move the control column back to the neutral position. This is simply not setting you a realistic expection of how to trim in reality as, in reality, you wouldn't move the yoke. This may seem like a tiny detail, but as trimming is such an important part of flying light aircraft this should demonstrate how doing your learning on a PC can set you up with misconceptions.
- This takes us very nicely to 'feel'... a sense that you are robbed of with a PC. You can't feel anything. 'Feel' helps you to maintain a balanced turn. It helps you trim the aircraft. It helps you keep the wings level on a sunny day. It helps you during the stall. Without 'feel', you need to compensate with another sense and that is almost always the sense of sight. This over-use of vision can lead to problems when you come to fly for real. To maintain a balanced turn in an aircraft, you co-ordinate rudder and aileron. You can 'feel' this through your backside and stomach as you will be able to tell if you are slipping or skidding. Not so on a PC, which means you might need to check the balance ball on the T&S Indicator a lot more than you would do for real... maybe developing an over-reliance on instruments and a 'looking-in' mentality. When you trim the aircraft, one trims until one feels no forces on the control column. Not so on a PC. Which means that you are trimming 'visually', observing (as mstram said) whether the aircraft pitches due to insufficient trim. This visual check should be just that -- a secondary check that you have trimmed correctly. The primary sense should have been 'touch', a process which isn't replicated on the PC. When minor turbulence (or a thermal, on a sunny day) lifts a wing I would normally react to the 'feel' of this before I react the the fact that the nose is drifting away from the desired heading. Maybe this is down to my gliding experience as well, I don't know, but I feel (no pun intended) that on a PC you are robbed of an incredibly important sense.
So... some thoughts of my own. Not an exhaustive list, by any means, and perhaps not even correct - they are based on my experience and opinions only. Nevertheless, perhaps it serves to reinforce the opinion that
learning to fly from scratch, on a PC, is not something to be done without consideration.
Take care
Charley
p.s. well done for reading my rambling comments, if you made it this far