I use a plain blue background ( R 0, B 64, G 0). I find it easy on the eyes, and I think it saves ram, so I like it even more.
I've had pictures in the past, even tiled backgrounds, but after a while I get really tired of them.
I always like a minimalist approach. But if I could I would modify Windows' interface and get a complete Amiga WB makeover, just for the sake of it and because oldskool Amiga rocks!!