Not a necessary play if you already played Portable Ops. Simply the same scenario without a story, just missions:/
If you haven't played the first game, then i believe you'll enjoy it more, but if you don't know which one to buy, don't even hesitate. Buy the first one.
Nevertheless, a few adjustments have been made relatively to the first game. It's faster do drag soldiers and easier to grab and choke adversaries.
Good gameplay, relatively good scenarios and also an interesting variety of weapons. Gives you the opportunity to use characters like old Snake and Raven. Doesn't take much time to beat the game though.