Have managed to get hold of a 405 - it is smaller than my 205 (still sligthly bigger than a normal watch). I will write a review once I've had more time to play with it but here are more my first impressions:
1) Using the touch bezel, menu options and sending data wirelessly to PC takes a bit of getting use to.
2) Much quicker at locating GPS signal.
3) Garmin Connect is a much better option for looking at training data instead of Training Center software. Need to figure out how to use it with SportTracks s/w.
4) Display is smaller than the 205 and can only show up to 3 fields instead of 4 on the 205.
So far I'am quite pleased with it - definately found the 205 easier to use to start with though.