I dislike having to transfer stuff between bags (partly because I don't trust myself to remember to transfer the right things), so I need something that will carry my 'core essentials' (keys, wallet, phone, iPod, travel pass, camera, Leatherman) for day trips, but also take my work gear (PowerBook, A4 document wallet, books, VGA connector, laser pointer, lunch etc.) for work. Because I walk home from the station (a 30 minute journey), and travel on a couple of different kinds of public transport, it also needs to be waterproof and comfortable to carry.
Leaving aside hand-held briefcase type bags which are impractical for my needs, there are two main styles of bag: rucksacks and courier bags. In the past, I've favoured rucksacks because they distribute the weight more evenly across your back, but they are a lot more awkward to get things in and out of. Courier bags, on the other hand, can be swung around to the front to quickly extract your keys, wallet or travel pass. I don't like to be awkwardly rummaging in public.
In the end, I got a Crumpler Cheesy Disco with a generous discount at the MacExpo. Despite the stupid name and frankly terrifying website, it's a very classy bag. It's beautifully made, with very tough material, buckles and zips. It has also remained totally waterproof after some drenching walks home, and has some very thoughtfully designed features. For example, an extra lip of material2 folds over when you close the main flap, preventing any water from sneaking in at the edges. Chunky straps of fabric which lie flat against the bag make good loops for attaching camera pouches to, but don't get in the way otherwise. Best of all, the padded laptop cell inside is self contained and totally removable. So when I take my work stuff out, I've got a very roomy courier bag. It looks as if it's made to last, which is just as well because I really don't want to set out on the One True Bag quest again.
1 And not just because I don't have thousands of pounds.↑