I had a TomTom Rider 2 which if you just want to do A-B routes with good mapping and good instructions then it's great.
I now had a Zumo 550, which has more functionality and what I disliked about the TomTom, it has a bread trail (records your track) so you know exaclty where you've been. Planning a journey is much easier too. However, I dont think the mapping and instructions are as good as TomTom.
For what I want then the Zumo wins.
If you want bluetooth get a 500 or 550, if not then a 400/450 should do.
The new 660 it's quite right yet...
More info here than you can read in a lifetime:
http://www.advrider.com/forums/forumdisplay.php?f=37
Nick.