I managed to squeeze in a lunch at Ocean Terrance on Discovery this past November.
And when I say "squeeze in", I literally mean the last available seat at the very end of the bar. It was packed throughout the cruise, or at least the times I walked by, so I jumped at the chance to try it out. Being à la carte pricing, I was surprised how consistently busy it stayed.
Anyway, sushi was good. Not the best I've had (spoiled here in Los Angeles), but was happy with the rolls and other items I tried. Yes, it's a limited menu, but I'll go again - it's a nice change from the other options, especially during lunch.
Oh, and like @lx200gps, service for me was also slow. There were only two servers during the time I was there and they tried their best.