For what it's worth, I installed Debian Wheezy on my ThinkPad T420 last week, after years of being away from Debian GNU/Linux (I got caught by the Ubuntu bug), and I must say that I was extremely impressed; my video card, sound card, ethernet card, webcam, microphone, trackpad, trackpoint, SNES and Xbox360 controllers all worked out of the box. I had to apt-get install firmware-realtek, a non-free package to get Wifi working, but once I had downloaded it, my wireless card worked without any further configuration.

As for stability, I think Debian's reputation for being rock solid is well established and deserved. I'm extremely happy with the setup I have and with how little effort I had to expand to get it.

If you have an older machine lying around, I'd heartily recommend giving it a whirl.