It's a medicore game, to be honest. The first level (I assume the demo is of the first level) isn't the strongest one. The game gets better as you go along, but not a whole lot better. If you really hate the demo, you probably won't enjoy the full game.
Just wait for Resistance 2 and buy that instead. If you're worried that you won't be able to follow the story, here's the origional's 'storyline':
-- The Chimera originated from Russia, and have taken over all of Europe apart from Britain. Britain has almost fallen too, though.
-- American troops come. Nathan Hale is an American troop.
-- All the American troops die, apart from Nathan Hale. He's infected with the Chimera virus (which makes people fall into a coma and turn into a Chimera).
-- We also hear about (but never see) the mysterious 'Cloven'. They are warriors who speak Russia, and seem to view both human and Chimera as enemies. (They eat people too.) They'll probably come up in the sequal somewhere.
-- Hale's taken to a conversion center (where the infected are converted into Chimera faster), and wakes up from the coma.
-- He goes on a killing spree around Britain, killing lots of Chimera. He also gets stronger and stuff due to the virus, and his eyes turn an amber colour.
-- He (and an awesome English soldier) discover that the Chimera's are digging up a large building that had been buried. This - along with their futuristic technology - strongly implies that they are aliens, as opposed to a Russian genetic experiment gone wrong.
-- Nathan kills more Chimera. In particular he kills a bunch of 'Angel' Chimera (think of them as Queen ants) which made all the Chimera in Britain freak out and die.
-- w00t, Britain is liberated.
-- Wandering around by himself, Hale is picked up by special-ops-looking guys and flown away in a helicopter. The end.
