Lots of people have found it to be a difficult sometimes tedious read, with the most common complaint being, "Too much about whaling!" If you're looking for some interpretive assistance, Tony Tanner has a great, easy to read, introduction in one of the editions. Google his name + Moby Dick and you can read it for free on Google Books.
Definately a good book, but I'm pretty sure it went over my head. In it we are told one of the most famous stories in the world, it's a book everyone should have read! Cons: Tends to shift topic and style of writing a lot, makes it kind of hard to follow the "plot"