I got a Joe Rocket sport bag from Santa this year. It splits into two bags. I normally ride with just the bottom half. There's enough room for wet weather gear, cell phone, extra glasses and gloves, bottle of water and a few other misc items. I've ridden about 1,000 miles with it, and I'm very satisfied. I've used the top half a couple times, and it also seems to work well.
The only complaint I have is that the lower half seems to have faded in the sun, and it looks mis-matched with the top half.