In the video above, the camera operator is doing everything that the subjects are doing (well, except for the cartwheel). The first 25 seconds of the video were shot without the Steadicam, the remainder is with it. The results speak for themselves and that's with just a little practice. And again, with more practice comes better results.
The iPhone quick-release mount pops out of the Smoothee fast, and has a flat bottom that can be used to stand the iPhone up on a flat surface. It has a threaded insert in the bottom of it as well, so you can put your iPhone on a tripod without the need for a separate mount. Additional mounts are available for the iPhone 3GS, iPod Touch (fourth generation), GoPro Hero and Hero 2, and Flip MinoHD, at around $25 each.
Regardless of how well it works, for most iPhone users the Smoothee is overkill. Its size and weight pretty much kills the whole convenience of shooting video with your iPhone and using it well requires more effort than just taking out your iPhone and shooting a clip.
On the other hand, if your iPhone is your primary device for capturing video and you are tired of just how shaky things look, get the Steadicam Smoothee.
Conclusion: Highly recommended
Though the Tiffen Steadicam Smoothee for iPhone 4 is a luxury, it's one worth having if you shoot a lot of video with your iPhone.