I have one installed and working.
Now that it's broken in, I think it's fantastic. (somewhere on this board somebody left some notes about how to break a trackball in, and it definately helped)
Sorry, can't compare it to happs, but I know I like it. It works like a champ, and I use it everyday, a few times a day. It's like being in an arcade, what can I say?
One nice thing is that the tolerance between the lip & the trackball is very tight.. so no pinched hands.