Okay, I've now done my second cruise with a partner from Cruisemates. Again I'd call it a success. We got along fine and neither of us ever bothered the other overmuch. There were a few of her habits I found moderately annoying, but then that's true of even my own relatives; and I'm sure I occasionally bothered her, too. Nothing worth complaining about, though.
I'd cruise with her again, although we were not as compatible as my last roomie and I were. This one was exactly my own age (versus 15 years younger) but had some medical issues that caused her to move slowly. I tend to walk briskly. Also, we had few interests in common; she was not at all into competitive activities, even non-active ones like trivia games; and I am not at all into spending time in the casino. We did both enjoy the piano bar, though. Most days I was up long before her, had used the gym, showered, and gone to breakfast while she was still in bed. We'd do our separate thing during the day, then meet and go to dinner, the evening show, and the piano bar together. That's more than the minimum "share the cost and stay out of each other's way" that I expect of a roomie.