My 2p.
Had 2 Milltek products, one being a Full Turbo back on my 1.8T MK4 Golf, and the other being a Cat Back on my Ed30.
Sound was fine, no problems at all, sound changed significantly with age, and only got better.
Fit was good, a disconcerting issue was the tendency of the Ed30 Exhaust to 'grow' when warm, the MK4 did not do this. I suspect the design on the MK4 alleviated this, I know it did expand, but was far less noticeable, I also suspect the MK4 was made from a more stable S/S.
Quality was a worry, they both cracked, the MK4 cracked circumferentially around the Turbo flange, and one of the box welds cracked fully. The Ed30 cracked around one of the boxes, on both ends.
Edit - added - I got both exhausts repair welded myself.
I cannot comment on their Cust Service, but having heard of others experiences on here suspect it to be very good.
Summary, a ok product,made by a company with good CS, however there are are other products on the market.
Would I buy another? Probably not, purely due to having both I have experience of crack, yes the CS is apparently great, but the nause of having to return your car to a dealer, or to Milltek HQ or arrange get it fixed yourself. IMO outweigh any benefits.
Just my 2p, based on my experiences.