I've been to it the last few years. It's basically a preseason game but between 2 squads of the celtics team. It's nice to see, but nothing too different than going to a normal preseason game. Some guys sign autographs near the sidelines as they do before regular season games, but there have been no formal opportunities set up to meet the players during past open practices. I don't think Pierce even played in it last year. I remember him showing up sometime after the game started in his warmups shaking hands around the sidelines like a presidential candidate, LOL. He then hung out on the bench the rest of the game.
Last year after watching the open practice I though Wilcox, Dooling, Daniels, and Jermaine Oneal looked great and were going to be huge for us during the year. Goes to show you how much these scrimmages mean I guess.