I dipped in at $20 and its my favorite game in the series.
Then again, I'm a fan of the series and was disappointed that Insomniac decided to cut it loose. Just when they started to get things together after 2.
I never played the demo for R3 but went in for the full game. Its definitely a solid game, its much shorter than the dreadfully long Resistance:Fall of Man, and you definitely get a feel of the post-chimeran invasion/post-apocalyptic America.
It becomes a last ditch effort to save your family basically. Universe seems very bleak and hopeless, with the few instances you help humans along your journey to your final destination: The Chimeran Tower at New York, to close the worm hole.
The story was pretty good overall, its much better than the other 2 games laid out but my main gripes with it was that it never went into the lore of the Chimera. Its basically a survival story and the Chimera become the weirdest hostile aliens you will ever run into. Theres clashes inbetween Feral and Regular Chimera and you will fight both along the way.
The ending (without spoiling anything) leaves a lot to be desired. And the way Insomniac left the series will leave you going "what the fuck? That was it? Really?" because you get so far of a great game to have a ending almost as abrupt as Mass Effect 3, and pisses off alot of people since they try to cover all the plotholes in one stone but it doesn't work because as I said it leaves you wanting more.
That being said, I believe the game has amazing replay value and if you are a typical fan of FPS that try something different with weapons and the environment then you will enjoy this game. Especially if you enjoyed Half-Life.
If you are looking for Modern Warfare or Battlefield esque campaigns, this isin't the game for you.
Gameplay focuses on using creative different weapons. Striving for new ways to kill, and Insomniac wants you to try these weapons instead of the typical semiautomatic rifle or Ak47 most FPSs hand you.
My personal favorite weapon was the Magnum and following up with its secondary fire. You can watch enemies splatter to bits.
So yeah, to sum up its a game definitely worth $10 for the Singleplayer experience alone. I never played the MP but the SP was well worth the $20 I put invested.