I also have a x220. A couple of the benefits/tradeoffs:
The keyboard is the best, I have never used a laptop with a better one.
Using a dual core sandybridge is slow but if you develop on a slow machine you'll never build something your customers can't use.
You can install coreboot on it and remove IME.
It is on the list of systems Qubes (and every other Linux) runs 100% on.
The screen is only 720p but you can install a 1080p with a mod.
12in is borderline too small a screen size but the laptop is compact.
An extended battery in it gets around 8hrs of use.
Almost the entire machine is magnesium, aside from a strip of plastic at the top covering the antennas.
It can actually run 1600mhz 16gb of RAM.