having owned an 04,05,07 and 09 models... i can tell you that the 09 model is the smoothest of the xb. not smooth enough to bore you, but certainly smoother than the previous models... as mentioned it has a different crank etc. the clutch is a little lighter as well.
big difference is the finish. the 09/10 have blacked out features. i prefer that as when you look at earlier buells you will see all different colour metal finishes. from the different fork bottom to tops, engine to case, frame and seat rails... they are all different. i think i counted 5 or 6 different shades. never liked that personally. hence why i have an 09 now.
but saying all that, somehow the earlier model has a better note! maybe just me, but seems to rumble more.