I will say, the Smart Case is noticeably more compact than the previous case that shipped with the iPad 3 / 4. That said, you're still adding some bulk.
I just picked up the smart cover last night and had one on my 3rd generation iPad before. Obviously not something that's going to give much protection if you drop it but protecting the screen and being able to position the screen for easier viewing makes it worthwhile to me.
Any advantages over the case?
Not sure I'd consider it an advantage but I didn't want the bulk of the case.