What about warranty? Would it affect your free service from BMW?

The warranty would probably be history. I would think the free service would depend on your relationship with your dealer.
Some might say that the law states they must prove the modification caused the problem however by the time you go through all the fighting and BS months have gone by not to mention legal bills etc. IMO not worth it.

I believe DMS will cover the cost of any warranty work that BMW don't cover as a result of the work. They reckon it's not detectable unless a dealer really goes looking for it. Or you could of course have it removed before taking the car in;)

What do DMS think the extra power does for the 0-60 and in-gear times ?

I would like to see actual dyno charts. I'm not saying the claims aren't true but I've modified a few cars and I have found that allot of companies claims are mostly hype. I don't know this company so I can't say either way.
The DMS warranty that will cover what BMW won't cover leaves nothing but grey area's. Most car companies will not honor any warranty claims for anything if the powertrain has been modified at all, you'll go back and forth between BMW and DMS until the cows come home.
The ECU's in these cars record almost all engine parameters so it's difficult to keep it a secret, not impossible but difficult

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.