I have a pair of Arkel T-42's with a couple thousand miles on them (well, at least one of them - I often commute with a light load in a single pannier). It's basically all commuting/utility (groceries, etc) runs.

They're very nice, and the outside pockets come in handy, but if I had to do it again I'd go with the XM-45's. The top loading style is a lot easier to deal with for errands and such. (my wife has a set of XM-28's that I stick on the front when I can't fit everything in the T-42's - big grocery/office supply/etc runs, etc)

They are reasonably water resistant, but definitely not waterproof; anything sensitive is in a ziploc. They're holding up really well so far, but again, I like the XM style bags better. If I had to do it again, I'd get the XM-45's.