I use a Deuter Race Exp Air for my daily commute to work (well tbh when the weathers good, I'm rather fair weather) bladder holder, lappy (12.5" model), spare clothes, pump, repair kit. odds and sods akll fit ok.
Obv I don't use a bladder with a works lappy in the bag (just in case it fails, plus it doesn't easily fit), I can't get a proper shirt to work without it being creased to hell, and there's no room for work shoes.
I tend to leave the work shoes at work, and get my wife to bring in a load of shirts at the begining of the week if i need to be smart, as she works at the same place and will come in the car after dropping off the kid.
Decent ventilated back on the bag, so a shower is only needed if I've been racing the bloody beemer and volvo drivers from the lights.
Love the helmet holder on it too, less clutter to hang onto getting into and out of the building (and to be entirely honest, if my hairs got to the needing lots of hair gel stage cos I haven't got it cut for a while I can cycle in, carefully with no lid, and then go for a good blast on the downs after work with a lid).
Umm can't think what else, apart from impressed with the quality of the consruction, and I like it.