We always have everyone gather around to watch, but then we never have more than 5 or so gifts anyway, so it doesn't take long (for those who are attention-challenged). It's nice for the giver to see the joy their gift brings and to get a thank you right on the spot. For those givers who really can't sit through it and continue playing, I don't mind, but I do make my child give them a thank-you-shout.
I don't really like it when the presents are not opened at the party. I like to see the smile when our gift is opened. And those times when they were just taken home to open later, I never got any kind of acknowledgement that they even got our gift. No verbal or written thank you at all, or even a mention that so-and-so liked the ____, and we didn't even see any of the presents at their house when we went to play. Even if you don't like the gift, at least show some some acknowledgement for the sake of the givers nice thought! But I guess "thank yous" are a different topic altogether.