A treat for the online FPS-ers out there. A rental for the rest.
Reason being is because whether you beat it on Easy, Medium, Hard, or Very Hard modes, you still get the same experience out of the single player.
The only reason one would go back after beating would be to collect the hidden Intelligence in each level in order to unlock certain cheats that you can activate at your whim in-game.
So if you're not planning to play this online at all, then do not buy it. Rent it.
Now, for the online FPS lovers.
BUY THIS. Definitely not a waste of your money. The online aspect has everything you want from an online FPS, and then some. There are customizable classes where you can pick your own set up of guns and side arms, as well as perks.
Perks are little "mods," so to speak, that really alter the way you play and the experience.
For example, you may have a choice of a UAV Jammer, which makes you invisible on the enemy's radar, or Stopping Power, which heightens the damage your bullets do.
If you choose the UAV Jammer, you're invisible. But your gun will also do noticeably less damage than it would do, had you chosen Stopping Power.
It's choices like these that really add a strategic element to the game, forcing the player to think ahead.
Along with the customizable class is a ranking system. As you level up with experience that you gain from killing people in matches and unlock more guns to use, you get closer and closer to that last level in each rank: level 55.
It's at this point where you're faced with the decision of starting over from scratch in order to gain a rank, or to just stay where you are. Starting over and entering into the next rank is called Prestige Mode, and it's brilliant.
You can do this 10 times before you reach the last rank, which favors the hardcore players.
But don't forget, online is fun. You don't have to do Prestige Mode, you can just get up to 55 and just have fun from there.
It's all up to you.