Here is my experience with Uber. My wife had a flight at LGA this morning. I reserved an UberX Car, that was supposed to come 20 minutes later. After 10 minutes I get a text that the driver cancelled and I will have to order it again. I tried a 2nd time and it said there are no Uberx available, only a Black car for more $. A few minutes later I tried a third time and is showed a car coming in 30 minutes. 5 minutes later I got a call from the driver that he just got out of the Holland Tunnel and can't guarantee the pickup time in Flatbush. I told him to forget and I drove my wife myself!!
If it is important to be somewhere on time make sure to order the car early. In Manhattan it will probably work better.