I dont have one yet, but plan on getting one when my Christmas bonus comes. I have a few buddies that do have the 320 though. They are all very happy with it. None have downloaded the Birdseye feature yet so I cant comment on that. However, the compass is a big improvement as you dont have to hold the unit flat like you did with the 220.Like the previous post said, they prefer the dc30s over the 40s.
With all of that said, their general consensus is that if you already have the 220, there is not really much need to upgrade. However if you dont have a tracker at all, go ahead and get the 320 vs looking at trying to pick up an older 220 for less money. They said while the new unit is nice, it is not lightyears ahead of the 220. So if you already have the 220 and are happy with it, no real reason to upgrade.
I myself dont have a tracker, so I will definitely get the 320.