DMS have a very good repuation in the UK, cars that have been tuned by them have been featured in several of the more repuatable magazines (like BMW Car) who have corroborated the performance claims. I have seen dyno results of cars that have been tuned by them on other websites (www.bm3w.co.uk being one) so I think they are fairly accurate. There is definitely the chance of getting stuck between DMS and BMW on warranty issues.

I am currently driving a 535d and haven't had taht chipped as the performance is great anyway but I will be getting the X5 chipped when I get it.