This is definitely not an easy place to find. Enter the Bottom Line bar on Front Street, go to the left (opposite side to the bar area) and walk right to the end. You'll see a small "Hidden Burger" sign on the entrance.
Now for the serious part - the food. I ordered the $10 lunch combo which came with a cheeseburger, fries and a drink. The burger itself was very fresh and obviously made with quality ingredients. The beef patty was well seasoned and perfectly cooked. The condiments were tasty too - a crunchy pickle, red onion and a slice of (real) cheese.
What I wasn't so crazy about was the fries - they were limp and soggy, and while I could tell they were fresh cut from real potatoes, the consistency just wasn't right. Either they weren't cooked for long enough or the oil wasn't hot enough, I'm not sure.
I would definitely go back for the burgers but not the fries. A solid three stars, only losing out because of the disappointing fries.