I know that there have been some posts about Alma already, but I thought that I'd put in a couple of thoughts.
I went last weekend. We did not sit on the roof, but the view was beautiful nonetheless. We also had some drinks at the bar, and, again, great view.
The drinks were fabulous - mojitos were minty and refreshing, and my companion's marguerita was powerful and tasted of fresh lime.
The food was not quite so fabulous, though it was passable. The chips were overly thick and tasted a bit stale. The sauces with the chips needed some salt, although they were generally flavorful.
The mole lacked depth and complexity. It was a little bland. Also, it was a huge serving of chicken with no sides, which I thought was a bit odd. We shared sides of beans and guacamole for a first course. The beans were pretty boring - you could have had them at any Americanized Mexican place. The guacamole was quite flavorful, and I would recommend it were it not overpriced.
I would recommend this place - for drinks. It is quite convenient to Red Hook, Cobble Hill or Carroll Gardens, and given the view, well worth while to go for a cocktail.