Purely out of curiosity, Sunshine, when was the last time you played on a setting below 9?
Judging based on save games, I had a difficulty 8 game I stopped playing on the 27th of december, and I think a difficulty 7 game I stopped playing on the 17th of december (or at least, these are the last save dates). Those are also solo games, my multiplayer games tend to be either difficulty 7, or difficulty 10 snake maps.
Really quickly, what I don't like about difficulty 7 is that border incursions are frequent, but minor, so you're always dealing with a nuisance here and a nuisance there, and as a result the most efficient method for winning on a 40 planet map (at least for me) is to
take every planet because there's no real strong resistance to doing that from the AI. So, I get bored mid/late game when I try to play the game "properly" and keep the AIP down, but just end up wiping everything off the map. I wasn't finishing most of my difficulty 7 games because they just simply got to be so boring and so much of a slog, and I'd been messing around with difficulty 9 a little with an eye towards some of the achievements, but then I ended up actually really, really liking difficulty 9. Difficulty 8, when I tried to drop down, seemed like a bad mix, where the AI
could kick me around, but only if it were so inclined. Difficulty 9, the AI is constantly trying to kick me around whenever I do something, and that's the kind of aggressive push-back response I'm looking for out of my strategy games.
Edit: A little bit on my other strategy gaming: I rarely play against human opponents because I like large multiplayer games, not duels, and finding a game like that online that's not stacked is impossible, so it's just not fun (also, too much micro needed when playing against humans). That being said, a while back playing just LAN games against friends I was more than capable of repeatedly crushing my friend's (and these were decent players) full 100 popcap armies with my 50 popcap Undead army (logistics was also part of it - rebuilding was much easier for me because I avoided upkeep and had a faster, more self-sufficient force by abusing the speed and vampire auras the Undead get, and I could get a force that was easy to maneuver and which I was comfortable with back in the field much faster than my opponents).
On the other hand, I do play against pretty much the highest difficulty AIs I can get my hands on. In regular Supreme Commander, I got to the point where I could go 1v6 against the hardest difficulty AIs, in Forged Alliance 1v4 I think it was against the hardest non-cheating AIs, in CNC Generals I'm pretty sure at one point I could go 1v6 if not 1v7 against the hardest AIs, and in CNC 3 I do decently against the second hardest AIs (though not with hugely stacked odds - CNC 3 is a lot faster paced and a lot harder to keep track of things than with Generals, and resourcing is... odd). Starcraft I haven't played in forever, but it had one of the best AIs I think I've seen in a strategy game of that sort, so even going 1v2 was difficult.