I have a DIY AT screen and I am very happy with it. For those of you who would argue that an AT screen limits your ability to produce the 'best' sound stage I would argue the exact opposite... depending on your definition of what the best sound stage is of course.
If the point of your home theatre is to emulate a 'real' theatre experience, then I believe the best sound stage is one that comes as close to replicating that setup as possible and that would be one where your speakers are behind the screen. If your definition of the best sound stage is the one that sounds the best, well then, I think it is clear that an AT screen isn't for you. I also think a home theatre is often made up of compromises and one way isn't more right than another.
I'll be completely honest and say I've never been behind the screen at a my local theatres but I have been to IMAX presentations where they turn the lights on behind the screen and show you the speakers that are there, so I am of the belief that the majority of the theatres I go to have speakers back there. With that thought in mind, it is my opinion that the best soundstage is one that can have the speakers behind the screen, where the voice calls come from the center of screen, not above it, not below it. Further to that your left and right channel sound is actually coming from the picture, not from beside the picture.
I went the AT for other reasons as well. One of which it allows you to use the complete width of your room, if you need to factor in the widths of your speakers when looking at screen size you are losing at least a good foot to a foot and half of width which could be used for a bigger screen!
For me, personally, I prefer the look of a theatre with an AT screen, I like not seeing where the sound is coming from, I think it helps immerse you into the picture more when your screen goes wall to wall and there is nothing but a screen in front of you. I'm really not concerned about what I paid for my speakers and don't need to be reminded of that investment so hiding them behind a screen is alright with me.
As far as degradation of picture quality and sound quality goes, I'm sure there is some, but on my $1,500 720p projector, does it really matter? Maybe if I was using a $8k JVC or >$10k Sim or Runco, sure, then I may have a different opinion, but for me, my picture looks fantastic. In regards to sound, again, I would bet a fair penny that every person who has ever stepped foot in my theatre would not be able to hear a difference if those speakers where in front of the screen or behind the screen. I certainly don't doubt there are people out there who could tell, I just don't know any of them, besides my room has a lot bigger acoustical problems than a couple of pieces of AT material in front of the speakers!
Blind tests are often encouraged around here for people to really test what they think they are hearing is actually what they are hearing, has anyone reading this thread ever had a chance to do a blind listening test with AT material, themselves, personally? I'd be curious to know if they heard a difference, I know microphones can pick up differences, but can you?
Sorry for the long reply