I've been using a Pelican case on my commuter for several years -- it's the best solution I've found for carrying my work laptop and dress clothes in Seattle winters.
Pelican Commuter Pannier by
joshua_putnam, on Flickr
The mounting system is from KlickFix,
KLICKfix Rixen & Kaul Carrier Adapter by
joshua_putnam, on Flickr
It also pops off easily to make me a seat on the train -- I have a 30-minute train ride in the middle of my 10-mile bike commute, and there often aren't any seats near the bike rack, or the bike rack is full and I have to hold my bike. But there's almost always room to use the case as a stool in the vestibule of the car.
The case is heavier than a cloth pannier -- I have various touring panniers that I used when I first started commuting this route, but wasn't happy with the way they carried my work clothes or laptop. (It doesn't take too much compression in a slightly damp bag to make durable wrinkles in a starched shirt. And the compression of the clothes in the bag made my laptop's screen fret against the keyboard, leaving keyboard marks on the screen.) The hard case leaves my clothes unwrinkled and holds my laptop securely without squishing the screen against the keyboard.
When it's heavily loaded, a hard case hung on the side can bend the stays on some lighter-duty racks, since the whole mass of the case moves together going over bumps. Not a problem with the Bruce Gordon rack on my commuter, but I did once bend the rack stays on my mountain bike using a side-mounted hard case for photo gear. (That wasn't the Pelican case, it was a Cobbworks Oyster Bucket,
