In my experience there is no perfect bag and I have about 8, so I can't comment on my partners fixation with handbags as a result.
I reckon 3 were really bad mistakes, rubbish straps, offering no protection to your valuable equipment and over expensive.
Of the new designs Think Tank are good, very well made and very waterproof, but like other makes I think they have too many silly pockets, so I loose things that I may need quickly, like passport or keys!
I have 3 Billingham, and they are rather heavy, but are nice and simple. I would say go simple and light, compare if you can with a lot of stuff in the bag and get one with a really good padded wide strap. Although all bags feel quite light empty they can be deceptive. I had a Crumpler, they are very protective, colourful, light, but really huge for the amount of space inside. I hate physically taking up more space than I ought and want my bag (and me) to disappear, especially when you are in an unfamiliar place.
I would always get one a little bigger than you think you need as the most frustrating thing is not be able to take what you want.