How realistic? Well, that depends on what you are trying to accomplish.
I can only talk about the Cessnas, since I haven't ever flown a jet in real life.
If you are talking about the actual eye-hand manipulation of the controls, and flight models, then FS has a long way to go. I think that, as far as actual hand flying goes, real airplanes are easier to fly then the simlulators, because you have actual feedback on the controls, and you can just turn your head and look, rather than have to mess around with switching views all the time. Also, the "ATC communications" in FS98 and 2K are severly lacking in realism.
But, if you are looking for a good navigation trainer, or instrument procedures trainer, then MS FS is great. The NAV radios, at least in the Cessna, behave almost exactlly as they would in real life. I have the Jepp airway manual pack, and I use those instead of the ones that MS includes (they are practically useless), and I fly airways, ILS approaches, VOR approaches, NDB approaches, according to the IAPs daily on the simulator to keep myself sharp on them. (FS2K is much better for this - many more NAVAIDS and airports) I set the visability to 1 mile and rain, and I won't let myself use the autopilot - It's great for developing a good instrument scan, and developing situational awareness. Also, it really helps you learn the charts, and how to fly the procedures (i.e., Feeder routes, course reversals, holds, procedure tracks, minimums, FAFs, MAPs, DAs, MDAs, etc...)
Jack @ AUS