I have done the same, though for some strange reason my Xbox360 pad (an official one as far as I can tell) actually has a standard USB on the end and requires an adapter to plug into a 360.
The great thing is that pretty much every game on steam supports it and even recognises it as a 360 Pad so game tutorials will display the correct labels and colours for buttons. Works great under Linux too.
What surprises me though is the number of people to whom this ability is surprising "You're playing a PC game with a joypad? What? how?"
It's also surprising that even gaming specialist manufacturers like alienware don't offer joypads as an option when building a system.
I thought the OUYA was a great idea after seeing the Kickstarter video, but I'm glad there are enough skeptics around to mention that pesky reality.