Uber is rather awesome, actually.
#1 - No money exchanges hands.
Boston cabbies are some of the rudest businessmen on the planet. Many times they pretend their card readers don't work or make their customers feel awkward for paying by credit. It is extremely frustrating and in today's world of connectedness there is no excuse for not being more "normal" in handling everyday business transactions. Outside of Boston many cabbies don't take cards at all (despite having a card reader in the cab). Personally, I don't carry cash and it is a bit of a hassle to "prepare" for a taxi ride.
If you ask a cabbie about this they will tell you that the cab company (the middle man you don't see) takes some 5-10% of their fare if you use a card.
#2 - Definite pickup time.
Calling a cab in Boston is a bit of a crap shoot. You call a dispatcher and you are told a cab is on its way and usually given a time estimate. The estimate is likely provided by a magic 8 ball based on actual vs. estimated.
#3 - Driver phone number.
Not provided by a traditional cab service as far as I know - Uber connects you directly with the driver. This is very helpful when trying to coordinate a street side pickup.
#4 - Professional
Some will value this more than others but all of the drivers are very professional.
I am in no way affiliated with Uber. I am just a pleased customer who is happy to finally have a reasonable alternative to the terrible Boston cab services.