If I can't locate a demo, I probably won't risk $60 on it. How does it compare to L4D?
I don't have a 360 so I wouldn't know about the comparison

but if i had to say its kinda like Fallout with a shed load of zombies. Here is a mini ish review that I've posted on Dead Island's FB page. Its about the story and characters for you to glance at if you like.
I won't give the ending away completely (even if it ended poorly). But it's clear that there's going to be a second game. But there are at least a few niggles I've got with the ending and other parts of the story.
The main story - Throughout the whole game you spend 100% of your time helping others out which involves bring other survivors food, water, weapons, etc in exchange for their help to get you off the island or some survivors give you cash, XP, or weapons. This at most times adds a meaning to the game and a great way to progress and level up throughout the story (even if the story is very thin and basic). But since completing the game its seems the many hours spent helping to save the lives of other survivors and time spent walking, running, driving across the map/island seems to have been pointless for a story point of view.
The other survivors on the island - There seems to be no mention or hint to what is going to happen to the other survivors who are located in the Church, Life Guard Post, Light House, Town Centre, Warehouse or Jungle which you have just spent 100% of your time throughout the whole game helping. The final cut scene doesn't give you the feeling of accomplishment like most games do these days, just that there is a hint and a nudge that there's going to be a Dead Island 2.
Character's Story - From the start of the game each player you choose will wake up in the same room and in the same bed after what seems like they have been on the piss all night. But in the intro video of the game you clearly see other characters doing different things throughout the video, so I couldn't see why the designers couldn't have just spent a little more time to work the characters individual parts into the game at the start e.g you see Xian helping a clearly bitten person on the floor in the lady's bathrooms which had a lot of potential to be a good starting place for her story line to start from. Maybe she gets attacked by the person she was helping and when you play as her you start in the bathroom. Also, the same random drunken guy in the video stumbles into and interacts with Purna, Sam B and Logan. They could have given the characters a more meaty beginning to the story.
The replay value - I would like there to have been some better unlockable features available after you complete the game, the only thing I seemed to have noticed at a glance is that you can play the game from the start with your current levelled up character, and all the inventory you finished the game with will be available from the start or so I believe. I would liked to have seen maybe a unlimited ammo or infinite cocktails/explosives or even a indestructible melee weapon but thats just my point of view.
Final say - I'm not going to rate the game just yet I will play through a couple more times with new players and see what that has to offer and try to look out for any major story lines that I might have missed the first time around that could give some evidence as to what happens to the other survivors or whats going to happen.