PMDG, PS1 & X-Plane,
I have the three of them - they all have their issues.
PS1 is really designed for 747-400 rated pilots, has no visuals and is extremely expensive (I paid €350 I think). That said, use it for a while and you could make a reasonable go at operating the real thing (in the sim). FMS/LNAV/VNAV & PFD/ECIAS simulation is superb and would make a brilliant FMS/systems trainer. One 'bug' I found was that at light load and TOGA is would break Mach 1 in level flight. Maybe it's not a bug! Joystick support is diabolical. Say no more.
PMDG 737 is good but could be better. Too much emphasis on graphics and not enough on the systems... accurate enough to keep a 737NG pilot in currency I would suspect.
X-Plane is in a class of its own really - flight model is second to none, you can design your own aircraft (which really teaches you about aerodynamics), instruments can be custom made (and show accurate VOR/ADF problems like dip etc), it's updated literally every week (I'm considering buying X-Plane 8 but it's evolving so fast that I might miss something). Systems simulations is generic and was non-existent - until last week that is - it's being heavily redesigned now. Even the scenery is spectacular.
Even now I look at FS as more of a game really (which the occasional gem) - stretching it takes a LOT of work, but X-Plane doesn't.
Who knows what lies ahead!!
V1R is correct, XP is approved for FAA ATPL training - says a lot about the quality really. I expect it to be used in PPL level training in the next couple of years.
As you can see I'm an ardent X-Plane fan
Edited to add: One provisio - to log time it needs to be used in with a full-motion sim in an authorised FAA training school. Just found out it's approved for the FAA IR/CPL as well.