I think this place is not very known. I just knew about it because I saw a serie called White Collar where in one of the chapters appear this place.
Well, I think is a really good experience go there. Is an island, so you have to cross over the river inside one Cableway. You can use the METROCARD (the same card that you use to get in the Metro). The view is amazing while you are crossing over the river . You have to take as many pictures as you can ! Then, in the island you can walk around and have another great view from the city. I think is really good idea see the sunset there. You will enjoy for sure!
PS. In this place you also can see Squirrel, like in many parks in Manhattan =)