Can pretty much second most of what's been said here already: very much useless for training aircraft handling, quite useful for training procedures and checklist flows.
Some personal suggestions for getting more out of it, both concerning fun and potential learning experience:
- Even though trying to gain actual flying skills by PC simming (whether MSFS or X-Plane) is about as useful as trying to become a match for Sebastien Loeb by playing Colin McRae Rally, having a joystick or even yoke with a throttle quadrant and maybe a set of rudder pedals adds big time to the fun. And while not actually making PC flying much more realistic, good add-on airplanes (and/or the modified airfiles by Alexander Metzger for the standard planes as a freebie alternative, see metzgergva.de) offer somewhat of a less arcade-like feeling (e.g., less unrealistic trim behavior which has been described above).
- Consider joining an online flying network (IVAO or VATSIM). Disregard any discussion about which one is better, go where there is usually more traffic in the areas and during the time you like to PC-fly (rule-of-thumb: VATSIM for USA, IVAO for Europe) and/or try both and pick your preference. It's free, it's not too hard to set up, and IMHO it really helps in practicing and/or maintaining radio skills and getting used to operating the aircraft and doing radio work at the same time.
For me, online R/T is one of the aspects probably most beneficial to real flying for the newbie and/or occasional private pilot. Practicing to stress-freely communicate in correct phraseology while going through the tasks involved with flying an airplane (even if it's just PC simming) has made me a lot more relaxed and completely stutter-free during my real flights. Online flying adds considerably to realism concerning airspaces and procedures, too, which are usually quite meticulously followed; you are also expected to have and use enroute and airport charts, even though of course they needn’t be completely up to date and/or approved for real flight operations. Besides, it’s loads of fun.
- For increased "seriousness" you might also want to consider buying FsPassengers. While you can of course crash your FS plane repeatedly without suffering any real harm, losing a virtual pilot whom you have flown to a decent "track record" (every flight is evaluated) and some "promotions" through your own bad decision at least gives you a small sting of frustration (at least it did for me when I decided to bust landing minima and flew a Cheyenne into the ground, killing my until-then successful pilot and having had to start a new career from scratch).
While of course not getting anywhere close to really going up in the air, I find these items to make sitting in front of your monitor and pretending to fly an airplane quite an immersive experience, and IMHO trying to flawlessly execute an online VFR (or even IFR) trip including flight planning and weather briefing (weather corresponds to real-world weather), correct R/T, navigation that respects all airspaces, correct departure and arrival procedures for the respective airports, satisfied virtual passengers etc. is quite an instructive and rewarding experience and nothing that can be achieved by simply starting FS and fooling around a bit. Besides being fun, I certainly learnt quite a lot from the mistakes I made during these armchair trips (luckily before seizing the opportunity to make them in a real airplane).