I went for the first time last night. The place was about half full when I got there around 7, and by the time we left at 8:45 it was almost completely full. Overall, poor service and mediocre food.
The wine menu leaves something to be desired. There were few options for buying by the glass, but what was offered seemed a bit expensive.
My SO and I ordered an appetizer of fried shrimp, calamari, oysters, shallots, and aioli. After we had placed our order, someone finally came over and brought us bread. When the app. came out about 20 minutes later, there were no oysters or shallots on the plate. When I brought this to our waiters' attention, he didn't do anything to rectify the situation, not even an apology.
We waited at least another 45 minutes until our order of mussels arrived. It came in a broth with prosciutto, caramelized onions, white wine, and garlic. In general, the flavors were alright. There were two mussels out of the bunch that were inedible. The fries were pretty crispy, which we appreciated, but they only came with about a tablespoon of ketchup.
There were two men sitting next to us who also had to wait about an hour for their food to arrive. A woman (who we later learned was the owner of Flex Mussels) kept coming over to see how they were doing and how their food was (probably a friend). Not once did she ask any of the other customers how their food or service was. Thankfully, the guys told her like it was: poor service and that a restaurant couldn't run this way. One of them had ordered the 1 lb lobster roll which, when it came out, looked like it couldn't have been more than 5 bites. He was shocked at it's small size, looked at the waiter and asked, "Are you kidding me?" He wasn't.
With food and service like this, we will not be returning any time soon. If anyone else has tried it and had a better experience, please share. Until then, I'd rather make the trip down to Café de Bruxelles.