I know this place is popular on the board, so I'll probably get in trouble for saying this, but their burgers were a huge diappointment.
After hearing from our friends and co-workers how good O'Sullivan's burgers were, we finally decided to check them out. We ordered two burgers: one with bacon and cheese and the other with BBQ sauce. Both medium-rare. Both burgers came well-done and completely unseasoned. The BBQ sauce burger did come with BBQ sauce, but the other burger had onions instead of bacon. I rarely send stuff back in restaurants, but this was just way too many calories to consume without enjoyment. It's not the onions that bothered me -- it's the terribly driness of the meat. So I thought I'll give them another chance.
After waiting for 5 minutes to have our waitress check up on us, I lost all sense of propriety (by this time I was not only starving, but frustrated), walked right over to her with my plate and asked her about the mix up. She was really nice about it and replaced my burger in less than 10 minutes. This time, the bacon was in place and the burger was medium-well, but still not medium-rare that I ordered. After removing half of the head of letuce that they stuck into an already vertical burger (this thing was as big as my head), and sprinkling some salt on it, it became edible, but still disappointing.
Maybe we got them on an off-night. Maybe going there on a busy saturday night is not a good idea. But our experience was so awful that I can't possibly bring myself to give them another shot. Life's too short to eat bad burgers.