I just invested in Alto 800 CNG LXI. This after researching the other competitive models from Hyundai, Renault and Tata.
The engine of the Alto is more refined as it is being developed for more than ten years now.
It is peppy and responsive to the paddle. But you may have to switch off the AC while picking up the speed. I have been driving an Innova prior to driving Alto. There was no issue of power in Innova. But Alto has power issues as the engine is one third of that of Innova. But that is what keeps the running cost so low. The car is so small it cuts through the traffic like a two wheeler!
Having said that, I won't advice this as the prime vehicle. It can be kept as a second car, if you can afford to, that is.