I have it, and I love it. I also have a 360 controller.
The best thing about it is that it is super customizable. Everything can be remapped to the nth degree. This means that it can work for games not intended for controllers, such as rts, turn-based, FPS (I know you can use regular controllers for this but they suck at it in my opinion), and so on. It does take a bit of getting used to though.
I personally also find it to be very comfortable to hold
I'd say it does everything a regular controller does just as well if not slightly better, and is superior to regular controllers for games that you'd usually use a keyboard and mouse for. After all, the stated intention is to make it possible to play dedicated PC games with a controller in the living room.
Whether you'd want one to replace your 360 pad depends on what kinds of games you want to use it for. If, however you don't have a controller already, then no contest, get the steam controller
It is an ideal replacement for PC gamers who dislike using console controllers. Whether you like it or not is not up to me to decide, but it's optimized for PC games (for the most part).
Be sure to put it on your list for Christmas, if you have spare money and time it's a worthy investment.