I got around to trying X rank Zones yesterday, played 6 matches, lost four, won two. My gut reaction would say these games have been harder, and they were mostly over in a good two-three minutes. Once a team got control it was hard to wrestle it back or lose it, and you gotta be ready to defend yourself.
Even with that win loss ratio, I enjoyed my time in the rank so far. It reminds me of that initial shock of getting to S+ in Splatoon 1. I went into the matches yesterday not having played much of the game for a few days, and now it's clear I should make sure to warm up before jumping into Rank X lobbies now. Once I got in the right mindset I was playing well enough to not feel like a total burden to my team.
Something that really shocked me, the lobbies filled up quite quickly. I was playing at an off time, but apparently there were enough people playing the game at that rank. I hope this means with time that the difference between S+ and X will allow for a better balanced solo experience in the higher ranks. The super good players can duke it out in X, while the normally skilled ones can play and improve in S+ without having too many pros beating them senselessly due to unlucky matchmaking.