Pesonally G-BPEC, I have found Fs2000 quite handy for procedural practise for my airline flying, and while the standard 737 and 777 are pretty poor in terms of cockpit and flight accuracy, I recently purchased a 747-400 panel from Phoenix Simulations which is pretty good.
This 747 has proper overhead panel, with most switches operational, accurate autopilot and FMC and pretty good flight dynamics.
I find Fs2000 - using the airliners anyway - is best used as a procedural trainer, I tend to leave the autopilot engaged for most of the flight and stick to management via the FMC and autopilot modes.
I can definately say it is a help when preparing for sim cyclics, and after having practised preflight procedures, high speed decents from varying points off the decent path, non-normal ILS configurations, I can say it has been a definate help there too.
As for practising for flying lighties, they are obviously a lot more hands on, and I believe there isn't really much to be gained.
Cheers...