The service is "safe" in the sense that you're unlikely to get robbed, but you're still playing Russian roulette with the drivers. The last time (and I do mean last time) I took a prepaid cab in Delhi, the cabbie decided to take a handy shortcut by driving onto the expressway up the offramp, into the wrong side and along the shoulder against oncoming traffic . Not by accident, mind you, but purposely, in order to shave off 2 km and 10 min off the trip (it wasn't even peak hour). Thankfully we emerged in one piece, but then it transpired that he didn't have the foggiest idea where to go and I had to navigate for him in the middle of the night.

Renting a car and driver costs more than a taxi, but if you go with eg. Hertz, odds are considerably higher that you'll get a sane driver who speaks a modicum of English.