I went to Velvet for the after party for Friendly Fires & The XX. It's a nice place, but kind of small. It's just one long room, with two bars at each end. It has a cool fireplace in the middle, located next to the dance floor and some small couches lining the back walls.
The music was great and the drinks were strong. The crowd that usually goes is more of the hipster and young urban variety. I think there might be some type of dress code, there definitely is a line in front that you have to pass through with the bouncers approval.
As at any place, be kind to the bouncers and they will be good to you. They definitely have a big influence on who gets in or who doesn't. I've watched pretty girls being carried out kicking + screaming for pissing off the door guy. On the other hand, I think they're super nice, and will definitely hook you up if they like you.
A good place to go if you want to drink with young and attractive hipsters that dress well, a bad place to go if you're not one of the above.