SeaDream is niche and one's response will depend on how one visualizes a 5-star experience. There's nothing wrong with it, but a lot of what we've come to associate with luxury resorts requires bigness, like walk-in closets, big bathrooms, lots of choices in restaurants and entertainment, structured programs. So, gut check time -- if your lux ideal requires big, then SeaDream isn't for you. Things like the wine list and the pool are going to be great but not big. Onboard shopping is very limited. This isn't a floating Bellagio -- you'll feel the boat move . . . a lot. I'd also say SeaDream isn't "plush" -- sheets, towels, furniture, fixtures are all very comfortable and up to date, but not over-the-top or to-die-for.
On the other hand, you'll love SeaDream (as we did) if your definition of luxury corresponds to what SeaDream brings to the table. The food is phenomenal (really) in creativity, presentation, execution, you name it. In this respect, smallness had no negative effect on choices -- the menu always had lots of choices, even for breakfast. These are 300' yachts, not cruise ships, which makes them both charming and convenient. There is wooden trim as well as doors and hatches that need to be latched. Literally nothing on the ship is more than a 2 minute walk from anything else -- I could go back to our room to retrieve a forgotten item and be back at the pool or topside restaurant in less than a minute. (Incidentally, wind and water make for good white noise -- we barely heard a thing from outside the room, even from the piano bar only 30' away on Deck 4 or the topside bar directly overhead).
Mostly, however, SeaDream delivers its kind of 5-star luxury through relationships -- both among guests and between guests and crew. (The latter may be more commonly known as "service," but it's still about personal relationships.) SeaDream is famous for its social atmosphere and its service, and I won't belabor the point other than to say it's real and it's all it's cracked up to be, and to add a few observations: