So a few weeks ago I picked up a PSP, mainly to play this game. In my book, it's not quite in the league of MGS1&2 (both are A+ games), yet it is much better than MGS3 (which I'd give a "B", due to the annoying camo/food/medical system and terrible camera angles). I'd say MPO is a solid (hoho) A-.
While I don't think this will convert any Metal Gear haters, I'm find that this is the most "gameplay" oriented of the Metal Gear Solid series (what a concept!); there are cutscenes, but none are more than 5 or 10 minutes in length. It took me a while to get into, do to the difficult control scheme in the action segments and the strategy/RPG elements.
In fact, I think this game should be practically dubbed a "shooter-action RPG" rather than "stealth action." A large amount of your time is spent recruiting new characters ("increasing your party"), and paying attention to numbers and statistics. (eg, checking to see if you should equip a character with an Ak-47 and put him in your battle squad, or rather put him in the medical unit, based on his weapons handling and medical abilities) There's also an aspect where you can dispatch spy units to different areas, which gives you clues where to procede next, and unlocks side missions.
Some other RPG influences I see: there is a strong emphasis on exploring every nook and cranny of every level, as to find hidden items and weapons. There are optional side missions. In some segments of the story, the cutscenes will be different depending on what you do.
In regards to the "action" segments, they remind me a bit of Goldeneye, if just a bit. Generally, you're given one or two mission objectives (eg, "blow up the supply trucks with TNT", "find the government agent and interrogate him", etc). You send out a squad of four characters after equiping them with the proper gear (you can toggle between any of them), and complete the objectives. The controls are fairly unique; I hated them with a passion the first few days, but then realized it was the best the developers could do with the PSP's limited control pad. The camera follows your character over the shoulder, while the analog stick controls the character. Pressing the "L" button locks on to the nearest enemy and recalibrates the camera, "R" is for first person view, and square fires (or is used for hand-to-hand combat and melee attacks if L is not held down). X is for ducking, triangle is the generic action button. Circle is for weapons/item selection and reloading, and the D-pad controls the camera.
These are my main issues with the "action" parts: it's too hard to be stealthy without the bird's eye camera. (which essentially throws stealth out of the window and makes these segments into a third-person shooter) Also, occasionally when you lock onto an enemy "above" you, and he remains off-screen while you shoot him. (ideally the camera should re-calibrate at an angle so that you can see him) One small thing: when you run out of ammo, your character automatically reloads it, even in the heat of battle, and you can't just switch over to another weapon. This got me killed a few times.
Also, there's online multiplayer. It's basically your standard FPS type multiplayer (deathmatch, capture the flag, etc.) with a Metal Gear twist. There are lots of cool things that make it worth while; this is the only deathmatch I've seen where rubber duckies and porn magazines are optional weapons! Other additions are using CQC and tranquilizer darts. One cool tie-in with the single player is that certain powerful weapons, items and characters are only useable in multiplayer if you've unlocked them in single player. It's fairly configurable, and shows me how limited Metroid DS really was in regards to multiplayer. (Apparantly, there's some weird online RPG mode as well, which I haven't tried)
In sum, while this isn't the best Metal Gear, it's very good, and stands head-and-shoulders over MGS3. It's probably the most re-playable Metal Gear game in the entire series (I've sunk about 35 hours into it and still haven't unlocked/discovered everything!), and the most "gameplay" driven of the series. (as opposed to story/graphics driven) I'd recommend it to Metal Gear fans first, as well as open-minded FPS/stealth fans, and action RPG fans. It takes a solid five hours to "get into" this game though.
Final grade: A-.
(Postscript: One of the cool additions to this is that the cutscenes are a semi-animated comic book, rather than done in-engine as in other Metal Gear games. I thought this gave it a very unique feel apart from other games.)