Ortlieb's TIZIP closure is the standard setter in waterproofing, and you could chuck this messenger bag in the river and still have dry clothes for work. There's ports for adding such extras as a sternum strap (one is supplied and you'll need it, as the bag is slippery) and inside there's, well, space for stuff.
You get a nod at some pockets but basically it's one big compartment. On the bike the bag gets better the longer you use it, the fabric gets less shiny and moulds round your back. After a few months you'll hardly know you're wearing it, and it's a super day-in-day-out workhorse. The silicon lube that the zip needs means that you have to be a bit careful getting clothes in and out, but that's a minor niggle for what is an excellent courier bag.