Panniers?

[abyssinia]

So since I got a new bike last May (a Giant Seek, and it's awesome) I'm remembering how much fun it is to ride when your bike doesn't suck, and I'm commuting by bike way more than by bus.

But grad school means I'm generally lugging around a laptop and books and other heavy things and, well, it gets kinda heavy. So I'm looking to get a set of panniers.

I'd like to find panniers which are waterproof, easy to put on and take off the bike, and have some sort of strap to make them easy to carry to my office/to class/etc. Something that converts into behaving like a backpack would be extra awesome. I'm planning to hit up my local bike shops this weekend, but I was hoping for advice beforehand.

Does anyone have any advice for features to look for/avoid? Materials or styles? Brands?

Thanks!