Poc Chuc is one of the few worthwhile BYOs in SF. It also happens to serve fantastic Mayan cuisine (think turkey). It's tucked away below Mission on 16th, which keeps the Marina away. Even Mission pros don't appear to know much about the Chuc. And here I am, posting about it.
Pros: Excellent food, inexpensive, friendly, BYOB
Cons: Can be busy (when there's not a depression on).