The wifi isn't great and it isn't park wide. There are specific locations that are shown on the maps, e.g. in Frontierland, right across from the Golden Horseshoe in the landscaping (look for the odd hitching posts). But it is better to have your own data and not be totally dependent on Disney's wifi. A CM told me that one big reason that the wifi isn't more powerful is because there wasn't any way to stop non-guests from hitching a free ride at Disney's expense. Since there isn't a password or code, anyone can use the wifi (and, apparently, during the testing period, everyone outside the berm was trying to use it). So keeping the power weak was an easy, but not so convenient, solution. Definitely have an extra battery pack with you. There are vending machines around both parks for power rods (pay once and get fresh ones as needed), but it's not hard to bring your own.