I'm one of the few backpack fans you'll find on bicycling forums. Probably only because I've been using backpacks for 20 years and see no reason to change my habits simply because I bought a bike. Probably because of my experience, I have no problems at all with the usual objections--it's too heavy, it makes me feel tippy, it makes me feel sweaty--and so on. I ride a hybrid anyway, so even the aerodynamics argument doesn't apply to me.

That said, the recent hot spell *did* make me wish for an alternative. Before next summer, I'll likely treat myself to one of these: The Arkel Bug is a convertable backpack/pannier. Wear it around like a backpack, attach it to a pannier rack for the ride. At 1400 cubic inches, it's a little small for what I routinely carry, so it will be a second choice for really hot days. I also doubt that I'd want to strap the thing on after using it as a pannier on a rainy ride .

Meanwhile, the Tom Bihn BrainBag is the best backpack I've ever owned. 2100 cubic inches also makes it the biggest pack I've ever owned. The three front compartments really help to keep things organized. The upper one is perfect for my multi-tool, lights and keys. The other two are reserved for work stuff and snacks.

The two main compartments are just plain gargantuan. I use one for lunch and for clothes (I can fit a complete change of clothes in there, including shoes and rain gear) and I use the other for books, laptop and work stuff.

Alternatively, either main compartment swallows up my helmet with room to spare. The bag is rugged enough that I can fill the thing with canned goods when I go shopping without worrying I'll leave a trail of groceries in the street. And I can fit a lot of canned goods in there. (I use it in addition to grocery panniers.)

Hope this helps!