It hogs too many resources on my computer. If it is downloading information everything else seems to slow to a crawl. Having realized this I disabled Genius, and so now I'm waiting for it to delete Genius data. And I'm subsequently amazed at how long that is taking! I mean, how much data have they collected on me over the past several months?!
So, if you are thinking about checking Genius out, I would advise against it, unless you really don't know what sort of music you like and what you should buy next. If you are in that boat, just come here instead! I'll be happy to tell you what to buy!