I've been twice...once just for fun when our Filene's first opened and then this year. It is crazy caotic, but if you plan on going after 10am, you'll still find tons of dresses. They aren't in any order.
You will most likely need to get the dressed cleaned and maybe fixed, but they had some great deals. Most dresses are size 6-12, but you can find other sizes. I'm a size 2, and threwout 8 racks packed of dresses I found 4 dresses. One was a $3k Vera Wang for $700, but I didn't love it.
My friend is a larger size and found several dresses she loved, but didn't end up buying one there.