Got it at 10am at Toys 'R Us on launch day (made my bro get when he had an hour off from classes). When straight home and played split screen co-op for 5 hours straight. Got to the large lake level about half way through the game. Went to work, realized the entire (obvious) twist of the game's story and texted my brother the secret. Went home, played another hour... accidentally booted ourselves out (selected Continue?: No) after Sheva lost a few pounds thanks to the friendly chainsaw maniac.
And developing a system of co-operation with two people is great. I play Chris, my bro plays Sheva. I've got the pistol, MP5, Dragnov, and Stun Baton with grenades. Bro has a better pistol, the Ithaca shotgun, the RPG-7, and Stun Baton and deals with a lot of the healing equipment. Inventory space is clausrophobic as hell, too. I'm usually pretty loaded with 3 guns, their ammo, a melee vest, and the stun baton -- taking up 8 spaces, leaving one for either health or grenades... kind of a pain. But meh.
Game is awesome, especially in co-op... even with split screen the game still looks amazing... people don't look like action figures like in MGS4... Also, buy the stun baton ASAP, that thing kicks ass and doesn't need reloading.
Oh, and about the demo...
If you played and liked RE4, no reason not to get RE5.
If you played and didn't like RE4, this isn't really gonna change your mind that much.
If you've never played RE4, the demo isn't that good of an intro and just realize the demo levels are abnormally hard and unforgiving. So if you like it but kicked your ass, just imagine it more fun and being somewhat easier.