Children under 3 don't need an Express Plus Pass. All others do.
At the moment, the $20 price applies to passes purchased in advance. Buy in the park and it costs $25. During "non-peak" times, the price goes down to $10. The next non-peak time begins after Labor Day weekend.
The Express Plus pass allows the bearer ONE express access to every ride in the park. The pass takes the form of a card with a magnetic strip which is swiped by an attendant on a hand held device, so they can keep track. If the attendant has no device, they will write on the card indicating you've taken that particular ride.
It cannot be used the first hour the park is open.
I was staying onsite, so I didn't use the pass. Everyone I've spoken to who purchased a pass was VERY pleased they'd bought it and felt it was well worth it.
If you want one, it's b est to buy in advance. The passes are date-specific (that is, they can be used only on one day). If you wait until you're at the park, not only are they more expensive but they sell out fast since only a limited number are issued (exactly how many is not disclosed). As word gets around, my guess is that most passes will sell before the date they're valid for.
Also, check out this thread:
http://www.disboards.com/showthread.php?s=&threadid=396685
