Interesting topic, as I too am awaiting arrival of a new 335D M Sport sometime in Sept.(Which
WILL be having a remap)
I currently drive a 330D M Sport coupe, I had the car remapped by DMS approx 14 mths ago and since having the work done I have covered approx 20,000 mls.
To date I have had no problems & the car delivers awesome performance with outstanding fuel economy & is a joy to drive on a daily basis.
All 3 companies mentioned in the poll are extremely professional & well worth talking to, however to be fair, you might consider E-maps as I have also heard very good reports about their work.
http://www.e-maps.co.uk/
As previously mentioned most people will go to companies that they have used previously & been happy with,others will be guided by forums like E90Post.
Perhaps Peppernick would be kind enough to post a link to his fantastic writeup on his awesome 335D (remapped by CA) as featured on EvoTechnik.