I used Tiffany once & they were fine.
The car was clean & they picked me up within five minutes of the scheduled times. Not a big deal for me, but there was a subtle pressure to make it quick at the grocery stop & the driver talked non-stop. The driver is the make-or-break part of these towncar services. You never know what you'll get.
Price-wise, all of the towncar services seem to be in about the same price range, give or take $5-$10.