[Edited 5/28]
Four stars.
A classic burger place. The burgers are large, plump, and delicious; there's a huge variety, too. They're not cheap, though! The fries, onion rings, and sweet potato fries are all likewise outstanding. Try the sweet red pepper relish at the tables; it's great.
It's almost always hopelessly crowded, though. There's often a long line to get in. Once in, expect to get jostled and bumped quite a bit. My seven-year-old son was hit once in the head by a chair that a waitress was moving, and bonked several times on the head by elbows (he's okay). Odds are you'll end up at the long central table, seated shoulder-to-shoulder and face-to-face with strangers if it's at all busy.
I almost forgot: be careful when you bite those burgers. They're much less flat than most burgers, and they're EXTREMELY juicy. There's a good chance they'll squirt hot juice all over your face or (if you're not lucky), your clothes. They still taste great, though.
I'd have given it five stars - I loved this place - but I have to admit that old Mr. Bartley really drives me up the wall in one particular way: he apparently hates Democrats. He's had pictures up on the wall of Democratic politicians (Bill, Hillary, and Al Gore among others) with their faces Photoshopped onto bodies wearing S&M bondage gear. His burgers are mostly "named" for celebrities and politicians, and the descriptions are rather slanted, to put it mildly. For example, one burger was called "The Bill Clinton - Don't inhale this slick burger".
Once a friend of mine got so angry at the gushing praise in the description of the Pat Buchanan burger that she grabbed a pencil and renamed it "The Bitburger" on her menu. Fortunately she didn't get caught.
If it weren't for the insane crowding, the politics, and the fact that I moved sixty miles away several years ago, I'd go there a lot more often.
Oh, another thing: parking? Don't even THINK about it. Bartley's is located in Harvard Square. There are a few parking garages, and their prices are sky-high. As for on-street parking, good luck - you'll need it! And watch out for pedestrians. Most of them are not only convinced that they own the street, but also apparently believe that they're totally invulnerable. You're better off taking the T, if you possibly can.
I almost forgot: there's no bathroom. In fact, you'll have a hard time finding a bathroom ANYWHERE in Harvard Square. Apparently Hahvahd students don't ever need to do anything so low-class as eliminate bodily wastes. Although I suppose they could always use the Harvard facilities across the street (not available to the public).
Actually, there ARE a few public bathrooms in the general area, but they're secret; you'll have to discover them for yourself. Or go to one of the larger restaurants which DO have bathrooms.
There's a small dish of parsley by the cash register "for fresh breath". It works really well, but occasionally I've found a grain or two of sand while chewing. Just an FYI in case you have delicate dental work.
All that said, a day spent shopping and hanging out with friends in Harvard Square punctuated by lunch or dinner at Bartley's was my idea of heaven for many years!