Shout's customer service isn't very good in my experience. I wanted to cancel an order a few months ago, and it took them almost a week to even reply back and cancel it. I'm also wary of shipping/packaging concerns like Chomp mentioned above.
I also don't like how they authorize your card used as payment at the end of every single month if you've pre-ordered something. Maybe that's changed since they revamped their site, I don't know?
If it's a really good "exclusive" on something, sure I'll use them again. But if it's only a mini-poster or something...I'll stick with pre-ordering stuff at Amazon.
"The good thing about science is that it's true whether or not you believe in it." - Neil deGrasse Tyson