I have used the service twice. The first time I sent two gold coins (a $1 and $3) in PCGS 'Altered Surface' details holders. Both coins had been puttied decades ago. The conservation service was able to remove all the putty and return the original surface. They are now in good holders.
The second use was submission of a 1932-D quarter, a 1928
Peace dollar and an 1883 No Cents proof nickel. The quarter was in a Whitman folder for decades and the back was toned flat grey and really ugly, and the dollar had just nasty toning and fingerprints all over it, but with really nice luster showing through. The proof nickel had a little corrosion on the reverse lettering. All three of these coins are on their way back as I write with good grades.
I am not a fan of dipping or messing with coins, and I actually love nicely toned coins, so I have been pretty selective with what I sent them. It is also important to read very carefully what kinds of problems they can address and what they can realistically do. So far I am happy with what they have done. I'll try to post pics. of the coins currently being returned and let you know then what I think.