Let's get the first major point out of the way -- this game is really quite easy. Right from the off, you have a number of superpowers that make it easy for you to fight and survive. And all you have to do to replenish health is "consume" other people ... any other people.
And this ability to consume others opens up quite a few gameplay options. For instance, whenever you need to enter a particular area guarded by the authorities, you can consume one of them and assume their identity. This gives you a non-combat option to enter areas.
That being said, though, combat is probably the best thing about Prototype 2. You keep unlocking new powers and ways to upgrade existing powers. Most enemies are cannon-fodder for your path of destruction. Occasionally, the game will throw upgraded, challenging enemies at you, but as you progress in the game, even these baddies are easily cut down. Consequently, combat is rarely challenging, but the animations and the viciousness coupled with all the powers and skills at your disposal make it quite entertaining.
I suppose all that's left to talk about is the story, which is really not very gripping. The flashback / cutscenes that provide exposition are quite messily done. You're better off listening to the dialogues or reading their subtitles to understand anything of importance. What they do reveal about the world is occasionally interesting, but mostly just serves to keep the game progressing and move you from mission to mission.
Missions are pretty similar, so it's good that the game is not very long. Even the side-quests rarely provide a different experience. Most missions can be categorised into 2 or 3 types. Very occasionally, a change of pace is provided by sequences in which you take control of a tank or a helicopter. But for majority of the game, you're killing bad guys and monsters and consuming them to replenish health.
In summary, Prototype 2 is a lot of fun. My disclaimer is that I've not played the original, so I have nothing to compare it to. When taken on its own, Prototype 2 is enjoyable, fun, and very easy to finish.