I also still have a T400 chugging along with Arch. It got put in storage for a few years until I dug it out while recycling some other junk and realized it still had considerable life in it.

I just set someone up with a T420 with a few upgrades and it's a fantastic value. Hardest part was finding a source for the rubber HD rails needed to swap a 7mm tall SSD in place of the original 9.5mm platter drive. It was technically a downgrade from a newer thinkpad laptop that they had, but the T420 is just a better machine from fit/finish/touchpad/upgrade/repair perspectives.

If anyone has advice on swapping the screen on a T420 please point me towards it - the T420 screen is, for me, the only negative.