Twixtor is outstanding - FCP 5.0's speed ramp is not
FCP 6 may have an improvement with it's optical flow plug in that is supposed to smooth out the shakes in shaky cam shots - and I've read - in Motion - offer smoother time remapping speed ramps.
Boris Red has a very nice Optical Flow filter as well as in their BCC filters as a plug in for FCP
After Effects also has a smoother time remapping filter that I think (like twixter and BCC) creates new frames and pixels to give it that speed up slow down car commercial look Andy