I love burritos, and I think Boloco on Mt. Auburn makes the best - better than Felipes. Apparently Quedaba (sp) is coming soon and will offer its burritos to Harvard Square, so we'll see if my opinion changes.
My first review of boloco was quite short; but I guess that's because it's a pretty simple place. Enter on the left, order and pay, then wait on the right for your food. The burrito offerings are creative and plentiful and not terribly expensive; the food is fresh; the servers are cheerful and polite and reasonably quick.
I had a large Cajun burrito with avocado. My only complaints: The avocado was right in the middle of the burrito, so I couldn't taste it in many of my bites, and I was still hungry afterwards, even though it was a large. Regardless it was quite tasty.