Also got one, and to tell you the truth, it's probably the component I use the less on my cp. But for the 60$ price tag, it's simply the best deal around. It work, spin ok, and you don't need any extra wires or USB controller board. It's a great all in one solution that work out of the box.
If plan to build a dedicated Golden T or a Marble Madness cab, then you should buy an Happ one. But for a 2 or 4 players panel, buy it!
There is also the Groovy Game Gear trackballs that start at 60$, but the price can go high really fast as it doesn't came with any buttons harness or USB interface. Sure, the overall quality of GGG products is quite superior. Dunno much about the Ultimark one, but they are probably greats as the GGG ones.