We got one for a week and loved it. It's really good to at least spend one day exploring the island. We had a blast. I believe it's $250 to rent one for a whole week. They switch them out and charge them up for you, and also come get you if you have a flat, get stuck in mud or break down. We used Monchos, next to the beer distributorship.
Is this your first time there? If so, i think only one day with a cart is better. use the taxi or water taxi as it will be cheaper and you will get a better feel for the island, local happenings, and "La Gente". They will pick you up at your request. also most resorts [caribe or caribe island where we are staying?] provide free bikes. Especially post hurricane the road past the cut will be dicey
Frank's right...the cart is cool if you're going up north of the cut, or you have a trip or a date planned that you would rather not pedal to for fear of getting sweaty; but a week on one will be just like it is in the States: everything will be cruising by at 20mph...what's the point of your trip?!
Daemyon, I'm not going to get into the topic again because last time it turned into a acrimonious free-for-all...but have you tried to drive a golf cart 20 mph on a San Pedro street lately? Your butt can get going 20 mph, but at that point the cart is bouncing straight up and down at 15.
I personally "wouldn't leave home without one", cart that is. We always have one, otherwise the ice melts before we get back from the Island Market.