It took me a few retries to get the PlayStation Store to not time out but once I did, this came in fairly quickly. First impressions are just holy crap this is cool. This is by far the best looking PS3 game and one of the best looking next-gen games period. Some incredible lighting effects, very bright, colourful environments that really pop out and unlike Heavenly Sword, shadows like don't look like crap and in fact look very detailed and fine. The audio is also very good quality though the guns seem a little understated. The music is done by the guy who did the tunes for Firefly and it's just excellent. It's hard to describe, you'll have to hear it for yourself.
The demo is relatively short (about 15 minutes if you aren't taking your time) but gives a pretty good vibe of this game. It definitely seems like it's going to have a lot of Prince of Persia/Tomb Raider type platforming mixed in with a fair amount of gunplay and in some cases, melee combat. The gunplay initially feels clumsy but gets easier as you go. You use the D-pad to select your weapon (one pistol class, one heavy class such as shotgun or AK-47 and one grenade class) and then you can either shoot from the hip or you can press L1 to go into a Gears-style over the shoulder view with a pronounced crosshair. There's no target locking and if there's any aim assist, it's very subtle. You can also press circle to get behind cover which works very well and never feels like it's getting in your way as it only does it when you ask. The platforming also seems a little odd as the actions you take when for example, hanging off a ledge are dependent on how the camera is situated at the time but I think I could get used to this as well (I did in Tomb Raider Legend.) The opening cutscene is full is pretty funny humour and even during battle, there's a few little funny lines (the guy does a Scott Bakula type "Ohboy..." when a grenade falls near him which made me laugh.) There's also a couple of swear words and blood which is interesting as I thought this was going to be a T rated game.
If the final game is like this, I'll be buying it for sure. The demo is polished to a mirror shine and this looks to be a stellar platforming game, as most of Naughty Dog's titles are. If you've got a PS3, you owe it to yourself to check this out. If you don't, the combination of this and Rachet & Clank would definitely have me considering a 40GB model.