Basically, you take any ideas you have about slushies from your childhood 7-11 and throw them out the window. Then, you choose a slush base from three flavors (ginger, citrus, or tea) and add a mix-in. These vary seasonally, but always look pretty fabulous and include things like blood oranges, lychee, and pink guava. You also have the option of adding a scoop of ice cream, or additional mix-ins. I think the ice cream would be really wonderful, but I didn't try it when I went.
I did try a ginger base with basil, and it was amazing. If you don't already know that you like ginger drinks, I might not start with it, since it's a strong flavor, but if you do, it's wonderful, particularly on a hot August day. I can't seem to find the price online, but I remember thinking it wasn't abnormally high for NYC, and that the slush was completely worth it. If you'd like to check it out, and I certainly recommend that you do, you can find their current location on their twitter.