I've logged over a hundred hours now and S ranked every mission, all teams at lv99. It's been a couple of months since I last played it so can look at it a bit more dispassionately. Now before you all jump on me, I'm a massive, massive MGS fan so I won't go into detail about how much I loathed the decision to put this game on a handheld, but just be aware I am a huge fan of the series.
Basically, pretty disappointing all round.
I won't go into the story too much because of spoilers and all, but I really wasn't impressed by it. It didn't really do anything for me and not even in the same league as the console games imo.
Generally, the normal missions were easy. Soldiers were very uninquisitive compared to the usual games and don't spot you till you're very close (except the snipers, they were fun). Also no difficulty settings unlike all the other games, instead it has the rank system where to get an S rank you have to rush through as fast as possible. I like to do things slowly and thoroughly, so I don't really subscribe to the idea of rewarding rushing through like a lunatic. Also you sometimes have to beat QTE sequences flawlessly first-time in order to get an S rank, when these come at the end of long levels and are particularly tricky (eg: the Boss fight reenactment) then that's pretty annoying.
Boss difficulty was generally fine, scaled quite well and it was satisfying whupping the harder bosses later on with the maxed out weapons. What was annoying was how all the ex-ops bosses were so similar to each other. So you just beat that APC, now do exactly the same thing again except this time it has a slightly different turret (which makes no difference anyway because you're doing it stealthily to get the S rank). And some of those missions were so long and tedious, capturing 38 soldiers from some of those APCs just took fucking forever, and one slip up and you have to start all over again.
What makes it tougher is that you're always battling the shitty controls. Don't get me wrong, it controls well for a PSP game, and is a hundred times better than Portable Ops, but it's still pretty awful compared to anything else. I played it through my laptop with a Dualshock controller and it's like a different game, glorious (though a bit laggy so not perfect either way).
The micro-management was pretty fun, basically the same system as Portable Ops with the GMP/R&D system added. Capturing system is almost a bit too easy but after all those hours I put into PO and its shitty "drag to the truck" system, I think I deserved it.
Huge range of weapons MGS4-style, but unlike MGS4 where you can whip them out whenever you want, you have to specifically choose them before the mission. As a result, I hardly ended up using any of them.
There is a apparently a whole bunch of stuff you can do in co-op, but since I know precisely one other person who owns a PSP, and they hate MGS, I was fucked. No, I can't use ad-hoc party before you start. Urgh I dunno, if you're going to have a large chunk of your game co-op only, maybe you should include an infrastructure mode or something? I'm guessing there's a fair sized group of people like me who would have liked to try the co-op but are simply unable to, so thanks for that Konami.
Eh, maybe I'm being a bit too hard on it but I really have few positive memories of this game. Yes, it's better than Portable Ops in every way, but that's not really saying a lot.