dcibrando: Disney Dining asks that you check in at least 10 minutes (or is it 15 minutes?) before your PS time. There will be others in the waiting area who also have PSs, so the restaurant will fill available tables with PS'ers in time order. In other words, if your PS is for 6:10 pm, then those with PSs for 6:00 pm who haven't been seated yet will be seated before you. (So don't get too concerned if others get seated ahead of you after you have checked in -- they might have PSs for earlier time slots than yours.)
Of course all this is probably affected somewhat by the size of your party, too. In other words if two parties have PSs for the same time and one is a party of 2 and the other is a party of 6 then it depends on what size table opens up. If large table opens up next then the party of 6 will be admitted first, if it's a small table then the party of 2 will go in first.
Our experience with PSs has been excellent. The restaurants we've been to have usually been running on time or even early. We always make sure we get there 10-15 minutes ahead of our PS time and many times we have been escorted right to our table. Other times (if the restaurant is really busy) we have not been seated on time. That's one of the big differences between a PS and a reservation: they are not actually holding a table for you just waiting for you to arrive. If the restaurant is packed, then they sometimes get a bit behind schedule PS-wise, but it's usually not too long of a wait.