As someone who spent nine and a half years at McGill I can safely say that their food services have always been lacklustre. It was much appreciated when they made all these new additions to them not so long ago. Quesada was by far the best of them with their fresh to order burritos. Prices are fairly reasonable and the food is good to boot. I always get a burrito but there are other offerings here like quesadillas, salads, etc.
There are three different sizes of burritos. I always get the medium-sized one as it is just the right size for me. You first choose your meat (or beans if you're vegetarian) and then you choose the other toppings for your burrito. My only real beef is that they charge $0.75 for guacamole when it would be ideal if they included it in the price.
Lineups are crazy at lunch so try to go either before the rush or after. If you're in a hurry they also keep some pre-made burritos by the cash. These are the burrito of the day and are only about $5 or $6. The obvious downside is that they're not made fresh to order and sit under heat lamps for who knows how long.