It's got horns as you can see, but other than that it's a streamlined case that keeps it simple. I've been using the case for some time now, and it works quite well. I'll be widening the button access points in the second draft, and making a few other minor tweaks. But overall - it's a really fun case that makes a statement - and for $15 bucks it's hard to go wrong!
I've tested this with an iPhone 4G. The 4GS has the exact same external dimensions, so it will work too.
When you put it on, insert the phone in the horned end first, then gently twist the case a bit and work the bottom in - one side at a time. Be gentle and you shouldn't have any issues.