I use a couple of the Xarcade 3" USB trackballs and I've been happy with them.
They aren't as heavy duty as the Happ controls, but they feel pretty solid. Takes about an hour or two of playing with one to loosen up nicely and smooth out.....after that it feels good.
My dad, my nephew, my kids, and I all racked up about 30 hours of gameplay on my cab when everyone was visiting for a few days this weekend. I never heard anyone complain about the trackballs.
Also, they function as regular 3-button mice, and I've got Player 1 and 2 buttons 1,2,3 wired up them, leaving those terminals on my IPAC open for other controls. They are also convenient as mice in windows or whatever OS you are using.