I installed FSX and X-Plane from steam when they were on special offer near the new year. I also bought a few addon packs to improve the scenery.
I would say that it is not realistic for GA aircraft, the landings are completely different (you just plonk it on the ground) and the planes seem to be far too stable, no real corrections needed.
For Nav the VFR scenery is dated and only really visible at high altitude. Things like roads, train tracks and rivers are in the wrong place, or randomly placed.
Basically for GA type flying it's not really useful, except as a 'toy'.
For jets it may be better, as the instrument rules flight bits may be easier to get right on a computer. I cannot comment though as I have not bothered with that.
It is a pity that in the time of google maps and extremely powerful PCs there is no realistic flight sims for the home market, but I guess there is no real market for them any more.