There are a lot of user reviews and field reports at http://www.backpackgeartest.org/reviews/Stuff%20Sacks/Dry%20Bags/ including Sea to Summit and Aloksak.

+1 on Sea to Summit being a good brand. I haven't used their dry bags yet (I just bought some), but every product I've purchased of theirs to date has been good kit. I bought the dry bags in case I ever slip and fall into a creek (total immersion). I want to protect my clothing and in particular my down sleeping bag. Wet sleeping bag = really bad situation.