What you see here is Weather Up, the Prospect Heights high-end cocktail bar owned by Kathryn Weatherup, an original partner in East Side Company Bar, with Sasha Petraske of Milk & Honey fame as a consultant. It's pretty, it's pricey, and it's not a place for a vodka tonic. Designed in a 'speak easy' style but functioning as a normal cocktail bar (you don't need to wait or call a special number to get in), the place has a tiled ceiling, an eight-seater bar, and a number of dimly lit corners. It also packs the people in, especially on weekends where early adopters have complained of long waits.
Reviews on the place so far from the regulars over at Yelp and Brooklynian are giving the place a mixed bag. On the one hand, regulars love having high-end cocktails in the nabe. On the other hand, there have been reports of long waits for drinks and an inconsistent product. Yet in the end, the posters seem in a agreement that it's a welcome addition to the neighborhood whether it will attract the cocktail hounds from the rest of the city or not. Further reading here and here.