It's accepted anywhere that accepts American Express ie almost anywhere.
We used it without any problem in November, and found it really convenient. We will definitely use it again this November.
The downsides are - you need to keep at least a rough record of what you have spent, otherwise you need to call Amex or go online to keep checking your remaining balance; you will need to take cash as well for tolls, street vendors in the parks; you are charged something like $2.50 each time you withdraw cash from an ATM (so we just didn't).
Another downside is that the exchange rate you get when you buy it is not great - but - if you take advantage of one of the free $ offers that are around in different travel agents from time to time, you can get an equivalent rate of around $1.86.
If you run a seach on this forum, there have been loads of threads about this in the past couple of months - too many to list - but they have had good responses, so try to have a read through those
