I only went from one airport to another LaGuardia/JFK.
It was a good way to get from JFK to EWR. Overall I thought that it wasn't really necessary to book in advance because you can just book a Carmel Limo taxi directly from the JFK airport baggage claim area, so the major benefit of booking through Viator is to lock in the fare. Note that the toll for the Holland Tunnel is not included in the Viator fare, and that you'll pay 14 for this. Add a 20 tip and you're looking at another 34 on top of the fare that you're booking. Not bad, but not an outstanding value for the money, just completely normal for this airport to airport transfer.
After arrival I had to call the local contact no. from my German mobile phone because all payphones in the arrival area of the LH terminal at JFK did not work. I had paid 93,93 via credit card for the transfer service. There was no info on the website through which I booked that there would be additional charges. When the driver came he first told me that I had pre-paid USD 84,-- and that I had to still pay in cash USD 29,-- in toll charges plus tip and this was expected to be around 20 of the total...... I was not really amused and I will not use this service again because there was no clear info at the time of booking what the total charges would be.