As a recent convert, I can tell you some of the best reasons to switch:
much better battery life - I am getting double the life
better screen - the screen is much clearer, the colors brighter and crisper
GPS - if you can get the upgraded OS
Bluetooth - I have not tried it yet but I am lookign forward to it

A couple of detractors
Its a little bigger - deeper so one of the car mounts I was testing would not hold it. It was also fittting more snug in my holder (I have a rugged pouch)
The desk stand in the Nextel box does not fit the new BB well, so it does not snap in

Overall, the two users in my area want them (They are on 7510).