I have 3 bags, depends on the weather:
rack trunk for winter - enough room for a shirt and lunch
grocery pannier for hot dry weather - I like to carry a nice bag into work and I like having the ablity to just drop in.
water resistant pannier for hot wet weather - need I say more?

Basically, the hotter it is the more official bike cloths I wear, and have to carry office cloths.

I've ruined my trunk bag and will be looking for a new one but I ruined my really cheapy panniers last summer so, I'm looking to replace them first.

I don't really like the grocery pannier for commuting that much. It is not aerodynmic...kind of like riding with a big square block on the side. I've been considering the kitty litter containers panniers or the duct tape panniers What the heck, if I'm going to have something boxy on the side why not almost free and boxy?