I have had one for around 7 years now and use it a lot. It's getting a bit tatty now (or "worn in gracefully" as Carradice would say) and I am considering sending it to them for some TLC, mainly the strap stitching underneath the bag, but it gets quite a hard life so I am not complaining.
It is bigger than a Bromptom small (S?) bag and smaller than a large T bag. I find it big enough for a three day business trip - I can fit laptop and related paraphernalia, clothes, wash things, waterproof jacket and D lock in it, but I am a fairly minimal packer. The two back pockets seem to swallow lots of stuff and are very useful, although you have to be careful closing these to get reasonable waterproofing. Overall the main bag withstands rain, but I find sometimes in really bad conditions water can come up from underneath.
My main niggle is the shoulder strap, which is usefully long for carrying the bag, but needs to be tucked away for cycling. I just do this by putting it on the top of the bag and closing the flap over it. No doubt someone will come along with some really neat solution to this, but I have just got used to having to open/close the bag to get access to the strap. I do like the retro styling of the buckles and straps but appreciate the quick release fasteners underneath. It means you only have to use the buckles to adjust it to fit more or less in there.