What's the best collaboration you've experienced? What was it about the fellow author that created something great? While I haven't written anything complete with another writer, I have helped him using my personal strengths and vise versa. It's been fun, and I greatly enjoy it.
I don't collaborate but I do work with a writing group and they're invaluable. Everyone takes a different angle - and in my case it's very helpful to have people of varied backgrounds, cultures, and writing styles looking at the book, because culture clash is a major theme.