I enjoyed MGS:PO for about 15 hours or so but after awhile the formula got a little stagnate.
The big problem to me is that there's no real variety to the gameplay. Each mission requires pretty much the same strategy. The recruiting aspect is fun until you realize that there's not a lot to do with the guys you have.
The boss fights were a mixed bag because of the controls. While its easy enough to sneak around and knock dudes out, actually trying to fight anyone is kind of bleh. And the part where you fight the spoiler (click to show/hide)
Metal Gear whatever in the hangar
was annoying because IIRC there wasn't anything in the gameplay up to that point that really educated you on how to play that fight.
So yeah, fun for awhile but was maybe 2-3 hours longer than it needed to be with too many stale environments.