I picked up the Happs 3" USB from PonyBoy (plus mounting plate) and the next day I saw some on eBay for a little less $$$ even after shipping, but I didn't care. I was getting a boat-load of parts from PonyBoy for both my cabinet project and a friend's cabinet. Shipment from PonyBoy was much easier and cheaper since it was one source, but that is another story.
To answer your question about the Happs USB, you are right on. The interface is built in, and all you do is plug the thing into your computer and it will (with XP anyway) autodetect as a mouse, and you can play around with it a few seconds after taking it out of the box. Pretty easy.
You will hear different stories from different people about how some trackballs are better than others. While I lived with what used to be a hacked PC trackball (anyone remember the "Crystal trackball?"). I thought it was cool. I could play Golden Tee fairly well (limit was the user not the trackball), so it worked great for $20 total. I never knew any better because I never played "real" arcade games that used a "real" trackball. So if you just want the functionality of it, you sure can go with less expensive (and sometimes "cheaper") products and be happy. Now that I am building my 2nd cabinet 7 years after the first, I am spending more time on quality, and after only about 5 minutes of use, not even mounted in my cabinet yet, I would NEVER go back to any other low cost trackball.
Again, everyone has their own tastes and tolerance levels. While I like my Happs, their might be someone out there that likes their Imperial just as much and possibly saved a few bucks.