I've only been on one Luxury cruise, with Seabourn, and I'm now considering booking a Silversea cruise. (with the new explorer ship in the Galapagos)
I must say I was rather underwhelmed by Seabourn, it was an Antarctica cruise, the destination was superb but I was expecting somewhat more from the service.
Having done several cruises with Princess, Celebrity, RCCL and the likes I thought Seabourn would have knocked my socks off but I think my expectations were too high. Yes, there was lot's of free alcohol and some free caviar but the service was so-so. eg. My roommaid was a job student. She was very friendly and helpful but also very often clueless and forgetful. Don't get me wrong, I still enjoyed the cruise but I left with the idea that an RCCL cruise with a full-blown drinks package would almost give the same experience.
Celebrity does also have cruises in the Galapagos so now I wonder if Silversea would give a better experience than Seabourn, otherwise I might just go for a Celebrity ship.