A guy on my team is a big sky ski rider & he picks up all of his gear on ebay. He bought one recently for $1,200 on ebay. He swears up and down on the sky ski over an air chair. He's skied pro with Sea World, so I value his opinion.
A tower will help you get more air, but is definitely not required. I've seen plenty of back flips, mobes (full twisting back), etc. on just a regular release. You can get some pretty wicked air on those things even without a tower. They're lots of fun.
And yes, like anything awesome, you can fuck yourself up on one.