I have this one (well, the older version) and LOVE it:

http://www.rei.com/product/710404

I have the matching Topeak rack (not necessary, but well worth it), which makes taking it off/on super quick and easy. It has zip down panniers, so you can use one side, both sides, or none. The top also expands. I use the main compartment for my lunch, with a cold pack in there. The rest of my stuff goes in the panniers or upper compartments. Yeah, it's pricey. But in my opinion, it's worth it--the perfect commuter solution IMHO.

There are lots of different sizes, etc. I have the big one and have never regretted having the extra space. I've had it for a few years and it seems super durable so far. Highly recommended.

Good luck.