Inexpensive but unexceptional
The Border Cafe has been around for a while now, and there are, in fact, now seven Border Cafe's in the franchise, which makes the experience a bit less intimate than it once was. It offers decent, filling food at a fair price. The kitchen is open later than many other local establishments. In my experience, their margaritas are overly sweet and somewhat watery, but there is nothing wrong with their beer. Better Mexican food can be found at many other locations around Cambridge, but if you're hungry, in Harvard Square, and there's not a line around the block, the Border is a perfectly reasonable option.