If it looks busy when it first opens, try again a little later in the set. I've found we get more interaction than usual with the characters at this M&G (perhaps because the characters are sequestered from the line?). We've done this twice and the first time we got Minnie and Mickey (and some really great shots of them admiring my fSIL's engagement ring); the second time it was Minnie, Mickey, and Goofy. It's 1 picture per account per day (we had four different accounts, so we got four free pictures)--plus all the shots go on your photopass account. You have to redeem your free photo in Epcot on the day it was taken.
If you're in California, it works slightly differently. It's at DCA in the morning (I think 30 mins after the park opens for 1.5 hours, but double check, it may have changed with the Carsland adjustments to the operating schedule). I think it's either Pluto or Chip and Dale. There you get a voucher that's good for *any character photo* on your photopass account from either park for a week. (So for example, we had 2 cards with us at the time, got our pictures with Chip and Dale on Feb 29, and on Mar 1 got a print of us with Goofy and a print from the Chip and Dale M&G in DL park).