Its mainly for the hardcore metal gear solid fans.
But I dont want to spend 2 hours on SMG, Assault rifle, and Sniper Rifle, Handgun is roughly 5 hours, and Shotgun took me 1-2 months to lvl up, now I know the key to lvling up SG is to Camp, but I honestly cannot sit in 1 spot for 5 minutes, I need action in the game. If it wasn't for my good friend, I probably wouldn't have leveled up shield.
Now the game modes are fun, some of them but not all. Now I love playing City and Prison, those are real fun games, but Prison is just horrible for game ruiners, Guards abusing prisoners, red smuggling in all their guns, and even people fighting over the Captain of the Guard Position, Like honestly who cares if your Captain, its just prison, this doesn't happen as much but it happens sometimes.
City is better with The Expansions because, I hosted a City game without the expansions and its was just turned into a TDM within 10 minutes, now with the expansions its not bad, some nuisances but not alot.
Training is not all that useful, because people want, Easy Grads, and all that stuff, and most newbies who join the mature games, piss you off and it gets annoying, and you need to be well experienced, and have Tactics that other people don't have, and who wants to sit down and plan for an hour how to train these guys.
TDM'S are probably my favorite matches ever, and are probably the best ways to level up skills, but probably the best things ever are Survival and Tournaments those is Real fun and is easiest way to gain reward points and Level up your character.
In Conclusion MGO is not as good as MGS4 Itself, many hardcore mgs fans play MGO now and probably not most PS3 Owners.