This game is BAD ASS.
1. The music is great. Totally early 90's stuff. Danzig, Faith No More, Alice in Chains, Dr. Dre, Young MC, etc..
2. Huge as HELL, but like all GTAs, the areas are limited when you first start playing, so I haven't yet seen the other areas of the game.
3. Stats are kinda cool and adds an RPG feel. Periodically you'll get stats.. like if you ride around in a bike and go in some pools, your bike riding stat will go up. If you run around, your "Fat meter" changes.
4. Physics engine is more realistic in terms of driving. Plays much better than the past games.
5. They say "fuck" a lot. Not really a big deal, but I'm glad they haven't held back in terms of dialogue. I know in GTA and VC the most you got was "shit" or "asshole". Makes it more controversial
As for dual layer, it's not. The game fits on one normal DVD-R (unless this one I have cut out videos or compressed stuff, but it doesn't appear so).
I only played for like an hour, but from what I saw, it's nice. I mean, it's nothing revolutionary, but it's definitely a change for the better. I haven't seen any of the more "detailed" features, like car modding or anything yet... but I'm assuming all that stuff comes wayyyyyyy later on.