Never had any work done by them personally, but phoned nms about a valve float problem on my biggish spec cvh and karl couldn't have been more helpful telling me what the problem was likely to be, and even told me a few 'trade secrets' i guess and sent me another set of valve springs.
Didn't have to give me any advice so to speak as i'd already built the engine and had a mapper for the omex, so wasn't going to spend big money with them.....but just that experience alone made me think if i'd had the money would have been a great place to take my car, as rs turbo wise they seem to be about the best
Tuners are a bit of a minefield and the rate bills can mount up is shocking, you need to meet them personally imo and not 100% listen to internet forums lol