My brother has a X220t now. He likes it a lot as well. As you can imagine it's way faster, very snappy and responsive. He complains about the resolution though. The display doesn't have as much real estate. That's his only complaint. It is a little ligther than the X61 Tablet even with the extended battery.
I was looking at T420 a short while ago when I contemplate buying another laptop. Some member here has the T410 and love it. The T420 is just a better spec'ed version of T410.