As far as sketchy-surprise factors, Atwater cocktail club definitely wins. I never fail to enjoy dragging my buddies down a dark alley and walking through a spray-painted back door. In terms of crowd, this is probably my least favorite bar in St Henri. On a busy night I can almost guarantee that you will be accosted by a drunken bro. One particularly memorable instance involved a strange man using my boyfriend as a human leaning post throughout the entirety of his drink. When asked to please lean elsewhere he thought it would be appropriate to argue how badly he needed to lean and "does it really bother you that much? I need the space.".
Aside from being a frequent receptacle for garbage humans, the service is consistently spotty at best. I've literally been abandoned for over an hour by the staff while they really should be trying to turn tables. If you're going to go, try and get a seat at the bar- proximity to bartenders seems to be the sweet spot here.
The big redeeming factor, and really the thing that has me going back (other than proximity and the space), is how cool the cocktails are. Incredibly elaborate and relatively costly, the taste and presentation of the drinks never fail to impress. I encourage any new visitors to order the smoke show (you won't be dissapointed).