I've had retips done by both Andy Kim and Soundsmith. I was happy with both, but liked using Andy a little better because he was local (at the time) and got the job done pretty quickly. Perhaps if I had a new copy of the same cartridge with the stock cantilever and the retip to compare I may have noticed some differences in sound quality, but I did not and I was happy with the way they sounded.
I've also had Van Den Hul retip and rebuild the suspension in one of my cartridges and was very happy with the results. It did take a looooooooong time, but it was rebuilt by AJ Van Den Hul himself. How cool is that? I bought that cartridge used and when it arrived there was no diamond on the stylus, so I never got to hear how it sounded "before". The seller gave me a full refund and it cost about the same as I paid for the retip/rebuild, so it all turned out well.
I've also had Van Den Hul retip and rebuild the suspension in one of my cartridges and was very happy with the results. It did take a looooooooong time, but it was rebuilt by AJ Van Den Hul himself. How cool is that? I bought that cartridge used and when it arrived there was no diamond on the stylus, so I never got to hear how it sounded "before". The seller gave me a full refund and it cost about the same as I paid for the retip/rebuild, so it all turned out well.