I don't dislike Genius. I don't think it really matters what tablet a person has as long as it works well and lasts long. But personally, I just think there's nothing to really write home about Genius tablets. They've essentially tried to copy everything Wacom has done, right down to colors and spinning wheels. The Genius PenSketch is a flat out copy of the Wacom Intuos 2, including the look of the pen and a practically identical pen holder: [link]
As for functionality/quality, for a long time Genius wasn't good at supporting many programs, including Photoshop. People had lots of trouble getting them to work properly. I think by now they would have fixed those problems, but I'm not sure. Construction-wise, I haven't held a Genius before, so I don't know what the quality is like, but I have held many different models of Wacom tablets. Except for the old Graphire2s and 3s, every one I've picked up seemed very high quality.
Among the different users out there, I think it's mostly a matter of reputation, software functionality, and looks.
No one hates their tablets, but on the other hand, everyone loves Wacom because it's the best. As long as it works, it's good. Wacom has a huge reputation and that's why nowadays no one even considers buying a tablet that's not a Wacom. Why I like Wacom and own one (Bamboo Fun) is because their drivers are compatibile with Linux and the nibs wear out really slowly on mine. I have my Bamboo for almost half a year and I'm using it every day for at least an hour, usually even more and the nib still looks the same. And I have 3 extra nibs. It's just because everyone knows Wacom rocks. No one wants to risk it with other tablets. Hope that helps.