We were there on Friday for the party and while we had fun, I found it quite crowded. I wish we would've known Small World would be running during the party; we waited in line for probably 20-25 minutes for it prior to the party, figuring it would be closed during the festivities. It appears they opened up additional attractions based on attendance; I think the carousel was also running.
Near the middle of the party, Space Mountain was listed at 30 or 35 minutes (we did it anyway; I'd say the actual wait was about 25 minutes). We waited probably 25 minutes+ for Haunted Mansion Holiday. There was a sizable line for Big Thunder Mountain Railroad; I think probably 20 minutes+. We did walk on to Buzz Lightyear's Astro Blasters, which was nice. (The ride did stop for a while, so I got a score of over 400,000!
)
Fantasyland overall was pretty empty late at night (we walked on to Snow White's Scary Adventures)--but that's generally the case shortly before midnight anyway, and Alice and Peter Pan's Flight were still quite busy. We got in line for Peter Pan's Flight about 11:45 or 11:50 and waited about 20 minutes. That was just fine, but then I was in Fantasyland the same time the next (Saturday) night, and the line for Peter Pan was roughly the same length. Splash Mountain was about empty for the party night, but again, it usually is quick to get on during chilly/misty nights. I imagine Indy's line wasn't too bad by the end of the night (we avoided it because it has single rider) and we should have thought about doing the Jungle Cruise; I hear that was walk-on. For my own encounters, the line at the Bengal Barbecue about 8 PM was shorter than I expected, but that's about it. I'd hoped that the big rides would be short waits, and that really wasn't the case for the rides I wanted to do.
(Now, in fairness, Haunted Mansion Holiday had a posted wait time of 80 minutes on Saturday afternoon, and Space Mountain got up to at least 120 minutes Saturday (probably because it broke down for at least a couple hours earlier in the day). And in fact, using FPs for Space Mountain on Saturday just brought its wait time down to about 15-20 minutes (Haunted Mansion had a sizable line for Fastpass holders too). Big Thunder Mountain was 40 minutes, though Fastpasses were available as late as about 10:45, I think it was. So those party lines WERE shorter than they were on Saturday--they just weren't anywhere at all close to "walk-on" or "off-season" short.)
I didn't get the buckets and buckets of candy that others mentioned, but I did almost fill my bag going through I think 4 trails. (EDIT: I finally LOOKED through my candy, and OK, granted, a bag almost full is a LOT of candy. It just didn't mesh with the reports I'd read that the bags they give you aren't big enough!
) Although, we could have easily gone around and done the Innoventions trail multiple times; great tip! When we went the Innoventions line seemed to be only going one way--I actually didn't see any trails truly double back and loop around. Except for Innoventions', they all had long lines at both ends (maybe not at the very end of the night, but when we walked through late, a few of the cast members seemed almost bored or annoyed, so I wasn't that interested by that point). Some trails were very very long lines going one way and then short lines at later stations going the other way, though.
We waited about 30 minutes to meet Jack Sparrow, but since that's a party exclusive that felt more than fair to me!
The best part for me was definitely being able to go to the park in costume, and seeing the others who did the same. A lot of cast members called me "Belle" and a lot of people liked my Chip purse. There were some really fabulous princess costumes too. I saw a lovely Giselle and then Briar Rose and I think it was Snow White with her; they all waved to me very cheerfully and princess-like, even though I was "just" in Belle's village dress. I adored the Tiana and Charlotte (I think the Charlotte was from the DISboards?). I can still see Charlotte hiking up her skirts and running near the carousel in a very Charlotte-y way! We saw a lot of other random costumes too, like Flo from the Progressive commercials, Bo Peep and a whole bunch of sheep, a Christmas Tree boy, a Peter Pan group, and SO many others I can't even think of them right now. One of the many pirates tipped his hat to my pirate friend; very nice.
I enjoyed it and I might do it again--though if I did, I think I'd probably want to take the 3 days off work I'd need to take and go to a Tuesday party instead of Friday. I just wish I would have had lowered expectations about the wait times and crowds. It didn't feel exclusive at all to me--I'd say overall the crowds were as high or even higher than the early afternoons of my previous two visits (early-to-mid December 2009 and early-to-mid June 2010, both at the start of a "high season"). During those trips I definitely saw wait times of less than 30 minutes for Space Mountain, even on weekday afternoons. So to be met with dense crowds throughout much of the party was hard for a planner like me. Especially when I'd promised five people that the party was a great idea in large part because all the lines would be much shorter than usual.
(Saturday was also a madhouse; I even saw the canoes running, and they did 3 Fantasmics!
But at least I'd expected that day to be crazy, so I didn't mind quite so much.)
I think details like this will help with people's expectations and/or deciding on whether to purchase party tickets. I think the party can be a mixed bag depending on what you want to get out of the party. 
We just kept rushing back and forth trying to get everything done before closing time! Thank you so much ^-^ The Bo Peep is actually a friend of mine too! The big group of pirates I believe are from Pirates for Hire or something like that 
