I've read a very small amount of fanfics. I usually avoid 'em either due to them being just plain uninteresting, cliched, chock full of crappy prose, Mary Sue characters, bad dialogue, etc. I've started reading some fanfics, but would stop and move on as they won't hold my interest. At all. Or the characterization is horrible. Or the prose is [parasitic bomb]. Or the dialogue is bad.
Every once in a blue moon will I come across a decent fanfic with decent prose that I will actually employ in my writing. That rarely happens, though. And I need to have suffered read through several bad fics just to find one good one.
For reference to prose and structure, you're seriously better off sticking to reading published, professional authors. Eragon doesn't count. It's just published fanfiction.