I have a Reva for about 4 years now.
On the positive side I would say this:
The ease of driving - it is like go-karting makes it a delight especially on crowded city streets.
Another plus is the quality and friendliness of the Reva Service Center Team at Bangalore. It has been of great help especially on occasions when the battery died and we had to leave the car by the roadside.
Low running cost about Rs 1/ km.
On the negative side:
The high initial cost - unless one is very eco-conscious or can’t drive a manual gear shift car(you have automatic options these days) I do not see a strong reason why one would go for a Reva.
Limited range - mostly around 45 km - which implies this can only be your second car for city commute only.
Battery replacement - Close to 3 years one of the battery(there are 8 units) started giving problems and had to be replaced(which again has to be another battery of similar characteristics). However problems continued even after the replacement. It became so unreliable that on one occasion the car stalled and had to be towed to the service center. Finally got all the 8 units replaced with a new set - costed Rs 70, 000.
Bottomline:
Do not look at Reva as "value for money" option. You can never recover the initial investment from the savings on the running costs. Further every 3 years or so there would be an additional Rs 70, 000 to be invested in new batteries. Also there is the Road Tax to be paid after5 years I think.
Look at Reva if you are highly eco-conscious / do not have a choice - a second hand one might be a good choice than a brand new one.