So far I am very happy with the performance (dual-core fast), form factor (compact), weight (less) and the screen resolution (same but smaller screen).
The other big news for this transition is that I did not take my GBs of items from C:/Data directory. I moved over completely fresh and I have only been moving over data that I need. This included Outlook .pst files which has made syncing with the Cingular 8125 significantly faster and more reliable.
Another big bonus is that I got all the "spyware" or "not need software" off of the M400 before I started using it full time. I was not able to accomplish this with the M4 which still has some AOL stuff I can't remove for some odd reason.
Only down side so far is the biometric finger print reader is very unreliable.
More to come ...