I have used Taxify about ~10 times in Melbourne, and twice in Sydney.
Service is fundamentally very similar to Uber (It's an "Uber clone" if you like). Drivers have been friendly and service generally good. The prices have been very good and often cheaper than Uber with their aggressive introductory pricing (as much as 50% off regular rates). I don't know how long the super cheap prices will last, but it's great while they do.
As with any new service, there have been a couple of times it has been hard to get a ride (no drivers, or the driver match is some distance away).
Between GoCatch, Uber and Taxify I rarely use regular taxis in Australian cities.