We've had a Milltek on our 182 for the past 3 years (replacing a pro-speed that was on there for a long time!), and it has been very good. We did notice though that there was a cracked weld on the outlet of the centre box when it was MOT'd this time. Thankfully, as the original buyer of the system, I got in touch with Milltek and was able to pick up a replacement section the same day (since they're just down the road) - and with that fitted it is back to sounding nice, but not too loud, which is just right for my missus (as it's her car really).
So although it was a pain having to swap it, I can't complain about the support from Milltek in swapping it for me without hassle.