www.projectorcentral.com recently did a head to head comparison of several screens including one of the DIY paint screen systems. They did not piss all over it, but gave it a very, very fair try.
I just want to say, that one of my friends did a DIY screen and he spent about two weeks making the drywall to display it on, mudding it, sanding it, repeat until perfectly flat. Then he Goo'ed it up and about a month after it was done and was perfect... the drywall got a small crack in it due to environmental changes.
I'm not a drywall guy, so I know I would never try it, especially considering that there are some good buys available on 16x9 permenantly tensioned screens out there.