While there is a degree of snobbery about this program, you'd be surprised at the number of professional pilots that have it on their home computers! I use it to remind myself of instrument procedures at airfields I'm a bit rusty on, and to have a look at the procedures at some of our diversion airfields.
For this sort of thing, MSFS is OK. No better than that - the ATC simulation is very poor and unrealistic, and most of the default aircraft do not behave particularly well. The palava required to change a radio frequency or select a VOR is a big distraction, and dreadfully unrealistic - and pausing the program while you find out where you are, relate it to the Jeppesen, reselect the navaids etc. is hardly a route to self-improvement (though the cat might learn a few new words!).
If you have one of the VFR scenery add-ons for your particular part of the world, you may find that you can reasonably simulate a cross-country route, though again the ATC and radio navigation aspects will be below par.
Some of the better commercial aircraft add-ons have extremely good handling characteristics, and very comprehensive FMC/FMGC representations, with prototypical databases and performance. However, even with two (or more) screens and a ninja computer, the fidelity to the real thing is limited by the fact that a) you're on your own; b) you've got no real switchery to play with and; c) the ATC and traffic environment is very artificial.
MSFS'X' looks likely to be a big visual improvement on FS2004, but I reserve judgement on the other aspects of it. It's likely to remain little more than a very complex, and very pretty, toy, unless you have the resources to spend a great deal of money on supplementary physical add-ons, and have a mate to operate in the other seat and can log in to one of the available virtual ATC environments. These last I have yet to experience, but I very much doubt they adequately recreate the manic nature of operating at LHR, JFK or LAX!
Scroggs