I've found FSX really useful for practicing flying. If I ever find myself in a position where I need to fly under tower bridge, compete in the Red Bull Air Race or land an overloaded DC3 on a remote strip in Africa I'll be ready!
I'm glad I saw this thread as I was wondering the same things myself.
I would be geniunely interested to know if anyone has used VatSim (real people providing ATC service). I was thinking of joining up to practice RT but I don't really know if it's any good, I'm a bit nervous that it's a step to far for a computer game if it isn't usefull. My housemate already gives me enough stick for turning the seat-belt signs on and off when I'm flying the 737!
One tip I do have is to turn the autorudder off in the realism settings as it makes control a bit easier/ slightly more realistic.
I'm lucky in that I live with a self confessed geek with a really good machine, playing in two screen means your not as likely to focus on the instruments as you can have a full screen for view and one for instruments/ GPS/ outside view.