Applications :: Genius Feature In ITunes - Exact Usage?

I've never even turned on the genius feature in iTunes, because I was put off by the link to the store. I have no intention whatsoever of buying anything from the iTunes store, though I do have an account cause I had to buy something else online. Here's my strong suspicion about Genius - that it's really there just to sell you crappy tracks from iTunes. And I have no use for that noise.

But, I like to keep an open mind. What if I'm wrong, and what if it's actually a cool feature? I remember not being sure about the shuffle function, since I like to listen to albums, so for the longest time I didn't use it. But one day I gave it a go, and what do you know. I liked it, for the most part! Here's one additional complication - I got like 50,000 songs in my library and I listen to all kinds of genres. Tell me about Genius - how do you use it?