I found this product to work great too. The missions are also a lot of fun. Admittidly I find it hard to optimize upto about 25 fps (although I do average 20 at the best of times), but MS are working on patches etc. Accleration also includes service pack 2, which makes further frame rate improvements.
Performance wise, it varies heavily from computer to computer, if you are using vista, the directx 10 option changes the way graphics are loaded and (should) give minor improvements, (MS say about 10%)
The trouble with FSX is that it was released ahead of its time, with most users unable to meet requirements, but MS couldn't win since if they left the gap between the next version too long people would moan then too.
Also due to the resources required, spyware etc on the computer can cause HUGE drag.
For Ref, my Specs are;
(Laptop), Windows Vista, 2GB RAM, 100GB HD (40-60 Partition Split), AMD Dual Core 1.8Mhz, 512 MB ATI Catalyst Graphics