Bought Dead Island a couple of days ago and I've played a few hours of it. Its basically Left 4 Dead in a sandbox environment with RPG leveling-up elements.
Quite a fun game, but I'm waiting for my usual co-op partner to come online to play it with me. Playing alone, the game can be slow paced and suspensful, whereas with a co-op partner (the game supports up to 4 players in a session) is much more frantic. So the game can be played either way, but my advice is to play it with a co-op partner, one that you can communicate with. You'll find the difficulty begin to spike just a little too high about 20% into the main quest line and you'll need a co-op partner or you'll find yourself being overrun and outmatched by groups of zombies.