I'm pretty happy with the range of difficulty in this game. IMO, unless the game is hideously difficult for one player, adding co-op partners makes things too easy. (Unless there's some sort of automatic enemy-scaling, ala Borderlands. Does anybody know?) I played Crackdown 1 on the max difficulty, and frankly, it wasn't that hard. Playing through it (CD1) with a friend is amusing, certainly, but it's pretty much a cakewalk, even on max difficulty.
CD2 seems better in that regard. I started playing on the middle difficulty level. It seemed pretty good at first: I managed to die a few times. But after I got back in the swing of things, and leveled up a bit, it started seeming a bit too easy again.
So, I started the game over again. This time on max ("Sadistic") difficulty. It didn't really seem any harder. Easier, if anything. Bummer. But the next day I continued my game and again(?) chose the max difficulty. This time, things were dramatically different. Enemies took a lot more kill'n, and they could do things like insta-kill me as I drove past them in a car! Apparently, the first time I chose the difficulty, it didn't "take".
So, yeah. For me, max difficulty in single-player is bit too difficult. I could do it, but I wouldn't be able to enjoy myself as much. Too much sneaking and hiding. Add some co-op help however, and it might be perfect.
I went back to the middle difficulty again, just because I wasn't sure what difficulty I had been playing on previously, and it does seem too easy for single player. I was able to go over to the Shai Gen island and kill a bunch of those armored gatling-gun guys pretty easily as a 3-star agent. Normal enemies can usually be one shotted with a crappiest assault or sniper rifle. You feel like a badass, but you're never in any real danger, so it's a bit boring.
I'm thinking that the next-to-highest difficulty may be perfect for single player.