Mixed reviews on both CKS and Mr. Samui.
HH Diamond and booked majority on points, plus a couple extra paid nights. Emailed & requested a villa in the low 300's month in advance. Arrived and wasn't upgraded any level at all. What was really frustrating: met someone who booked base room on some travel website, didn't request a specific villa, yet received one in the 300's the day before. Also, we selected our Diamond toiletries amenity or whatever it's called at check-in, never received it.
Nice villa, great views and private pools, huge shower & bathtub; room needed some TLC though (paint, scuffs, oil marks, warped floorboards etc.) If you like beaches, this isn't the place for you. If you like hanging at your own pool, that's what most people do. Good variety of restaurants at different price points, though still 'expensive' by Thailand standards since it's a resort, but not insane prices like the Maldives. Good variety of daily activities (tai chi, yoga, water aerobics, volleyball, muay thai, kayak, etc).
In the end: glad we went once for the views/pool villa, but probably wouldn't return, especially not on paid rates at the prices they're asking. Probably still a good use of points. Diamond recognition was pathetic compared to experiences at many other Conrads. And for those comparing the two resorts specifically: Conrad Maldives was a much better experience, and that's even factoring in that you spend more out of pocket there.
On Mr. Samui -- One of his family members was waiting on arrival, stopped at Tesco for us, was great on that first ride. The pick-up for departing flight was a disaster. 30 minutes go by after our scheduled pick-up time and no communication. Hotel finally got a hold of him & he said a driver was almost there. Another 10 minutes, still nothing. Called again and he said driver is 15 minutes away. At this point, we're going to miss our flight. He apologized a lot, but it almost screwed up our whole itinerary of intl connecting flights. Took Conrad's car service which then had to drive like a maniac on the less than stellar, curvy roads.