And the winner is...

aNobii!

Shelfari might look pretty, but it doesn't have the organizational abilities of aNobii. I can mark my books as finished (with date completed), reading, not started, or unfinished. Global editing is easy. Tagging is a breeze. There's a ton of sorting options. It warms the cockles of my little librarian heart.

Now, I wouldn't use it for cataloguing my complete personal library because it doesn't have some of the more powerful applications and tools, but that's why I'm a charter member of Librarything. But for compiling booklists, and especially for keeping track of what I read no matter who owns it, aNobii is the one for me!