Cylinder & Slide does good work. However, the waiting list is rather long. I had them work on one of my SP101s a couple of years ago. Here's how it works: first, you call in your order and they give you an order number, then you wait for a long time (7 months in my case), then they call you and have you mail in your weapon, then you wait (5 weeks in my case) then you receive your weapon and see that they've done a real nice job.
Not that I am particularly impatient but Marc Morganit (see my first post) has worked on two SP101s for me and done just as good work as Cylinder & Slide but with not waiting period and a two week turnaround. Also Cylinder & Slide is way overpriced compared to Marc Morganti.
Less expensive and quicker turnaround for the same quality work always wins in my book.
Also, I agree with the poster abover who said that if your weapon is new you should shoot it a lot before sending this in. I see at least two reasons for this. First, after firing 500 rounds of .357 through your GP100 the action will have improved tremendously and you may no longer feel the need to spend $$ on gunsmithing. Second, the gunsmith will be able to see the wear patterns on the different parts of your weapon and this will help guide the gunsmith's polishing, etc. of parts.