So far, Flight runs very smoothly. No stuttering or intermittent lock-up/pausing. I'm feeling very immersed at times with this new game (no doubt it's a game). I'm actually enjoying the heck out of it so far. So much so that I forked over the cash and installed the Hawaiian pack with the RV-6.
I've heard that 'Flight' relies more on your graphics card than it does the CPU. If that's the case, I'm now seriously considering replacing my old GeForce 8800GTX with something a bit more updated. I've thought about it in the past, but never took the plunge as I didn't think it would help some of FSX's ills (primarily stuttering).