Eficient way to get to Manhattan. Esp. if you are staying out of the normal tourist areas. We were staying on Stanton, which is below Houston ergo well below 1st Street. The fixed price is a better option in these circumstances than a cab and the cab drivers would be unhappy with a fixed fare to that location. You do get a bit of a tour around Manahattan as the other passengers are dropped off typically around 42nd St so it provides a useful orientation of Manahattan.
Highly recommended if you want to save on the major attractions!
I didn't experience jumping line stuff. And i think one of the top of rock and Empire building is enough and you don't have to go both. Although Moma sounds popular but it's actually waste of time. I like metropolitan museum though.