The prob I have with mine is the immense cranking power, you can turn over until they pass the point of accepting the normal charge. I had one on my 1.9gti and it had a clogged fuel breather so didn't want to start, once unclogged it started fine but the battery caught fire as the alternator tried to recharge it. The replacement was free but I have used it a lot and if it drops below a certain voltage you need to tease up the voltage slowly with other charges before a full charge.

They are the cheapest way to go faster though. 20kg saving in weight is better than a lot of horsepower.