Had a chance to try the vaunted burger at the Bristol Lounge recently. Verdict: you get a lot of service for your $18, a nice view of the Public Garden, a white tablecloth and good tableware.
The burger itself is not as extraordinary as I had hoped, however: while cooked to order, and served with decent fries, its roll did not stand up well, and it seemed a little overmanipulated. I don't think it compared favorably with similarly-priced burgers at Union and Caffe Umbra, whose patties seem to have better texture, and which are definitely accompanied by better frites. Not a crushing disappointment, but given its reputation, a little underwhelming.