Someone said that you tend to meet people pretty quickly - and it's true. I sailed on a smaller ship, but it seemed like by Day 2-3 - I had started seeing 'familiar' faces, and had folks to hang out with - IF I wanted to!
I did one excursion through the ship, and did the others independently.
I've paid the 'supplement' both times - it's still the cheapest vacation option going, and having a roommate - esp. a stranger - would totally warp the solo/chill vibe I have going on once I'm aboard.