I had a seagull medium that I wound up selling. They're fine if you just want to carry a few books, your tools, and maybe a change of clothes, but I found that when I filled it to capacity the strap really choked up on me.
However, I can see just from looking at the site just now that the design has changed: where there was a somewhat absurdly huge cam buckle there's something much more sensible. It used to be that, when the bag was completely full, I could just put my chin to my chest and it would be touching the big metal buckle, which I thought was no good (and apparently they agreed).