It needs to be big enough to hold work stuff (A4 pads, lunch, diary, etc) but be easily carried - I do the school run and generally have to help DS lug about sports kit, swimming bag, cricket bat, etc.
I like rucksacks but they arent smart enough to meet clients with.
So - big, sturdy, easy to carry with my crappy back. Up to £150.
I recently brought a Martha Stewart bag from Staples which is sturdy enough for files etc but still looks stylish. It was reduced from about Â£50 to Â£19.99.. it comes in walnut or black. sorry l can't link...