I've had a few burritos from Mission Burrito and have never been disappointed. The chicken burrito is my favourite. They offer various spice levels, but even the hottest isn't that hot. The burritos are prepared fairly quickly and the prices are very reasonable.
I have yet to figure out their hours so getting a burrito from Mission Burrito is a game of chance. I'm very impressed with how the single employee handles the stress of a line-up.