My experience has been anything but positive with GrabTaxi (it's called MyTeksi here in Malaysia). The biggest issues are:

* Since you need to specify your destination, taxi drivers can easily ignore you ("too far/near/congested").

* No rating system. This is Big Deal, as there's no incentive for the drivers to perform their best. * More expensive than Uber (not sure in other countries, but it's true here).

* The app "feels" heavy, but this is subjective.

I initially welcomed them when they first launched, since it's difficult to hail taxi in Malaysia, but I was met with disappointment.

I've used it once or twice in Thailand. It's your pretty standard uber-esque app that uses existing taxi infrastructure.