Funnily enough, I don't use FSX that much - simply because I can't run it as well as I'd like -
YET. If I get the performance improvement I would supposedly get if I install Windows 7 in 64-bit, I might actually change over, I just don't know that I want to install Windows 7 BETA (or GOLD) yet. I think I may wait until I can get a DX10 graphics card and have time to move my stuff off of the C/D physical drive and do a complete reformat and repartition of that drive. That may mean that I actually just go and build a new system, but we'll just have to wait and see as I don't have that money right now anyway.
I think that's what holds back a lot more people than anything. They got a computer capable of running FS9 well and FSX decently, but they don't feel that they're getting the performance they want out of FSX and that's why they haven't moved completely yet. I think the longer we go and the more that get around to their own personal "update cycle", we'll continue to see more movement to FSX.