I think that the Circle Line is one of the best way to really see New York City - what better way to view the city than to see it from the water? (Okay, a helicopter would be a great way, too, but a lot more expensive.) The Circle Line is affordable, comfortable (indoor & outdoor sections to account for weather problems) and gives you the peace of being out on the water.
I have been on both day & night cruises and have enjoyed them both. It would be fun to have a party on the boats...booze cruise with your friends around New York harbor ;)