I love audiobooks, but I really only listen to them if it is a book that I've read myself first to make sure I don't miss anything. Like pipergale said above, some people prefer visual and some people prefer auditory stimulation. I prefer visual, and I love making up voices for the characters in my own head first. I think that the imagination is more powerful than anything you could see or listen to.
However, if I want to reread a book, I like to listen to the audiobook version instead. I love listening to the ASOIAF audiobooks while I'm at the gym, and I don't have to worry about missing anything because I've already read the books!
In my eyes-or should that be in my ears?-audiobooks are really at their best when you've got someone who is good at acting. They need to be able to act in the different roles of the characters to an extent, changing their voices and mannerisms to suit not only what is currently happening, but also the personality of the characters themselves.
For example, you wouldn't want to listen to someone just read the lines of a character who is meant to be angry and judgemental, would you? You want them to be that angry and judgemental character, and for their voice to reflect that accordingly.
I agree with this wholeheartedly. A bad reader/actor doing audiobooks can ruin the experience.