I have a viper, and I'm very pleased with it. I've had no problems with it in the ~9 months I've used it. I usually cave with it on the lowest setting (which is fine to cave by), only turning it up to admire larger passages. A friend bought one at the same time, and vipers can be used to illuminate reasonably good photographs without the need to faff around with flash guns. His has also been used on a 2 week expedition without any problems, getting irregular charges from a car cigarette lighter (compatible charger is included in the price).
If I were looking to buy a light now I would seriously consider a little monkey light (~?100 cheaper, similar size and weight, but it has moved the electronics to the headset which means that spare batteries / AAs can be used).