The X60 is a good option, too.
Looks like the hardware support situation haven't improve a single bit since the last time I looked into coreboot. That's a pity.
How well does the computer work, when it is supported? Are there stability issues? Is performance the same? Do all of the same devices work, as with proprietary BIOS?
When it is supported, there are different outcomes. The individual board pages on the wiki can offer more details. The X60 is especially well supported because of a combination of factors:
1. Lenovo was actually pretty helpful
2. Lots of coreboot hackers have worked on the machine for years
3. Lots of other linux hackers have also worked on various parts (which improves the support for all the devices)
So without doing any research, my understanding is that the X200s and to a lesser degree the X230 cannot achieve that level of support, mostly because it would take the coincidence of all that work coming together. But it's not all bad: the Chromebooks are getting that work done by a group of paid engineers, and there is a recent addition to the supported laptops (not fully open, but close; not amazing hardware, but decent) - the HP m6-1035dx.
I got an HP m6-1035dx on ebay, and my experience is that everything works fine. I suggest throwing away the mini PCIe wireless card it comes with and putting in a better one, but I haven't brought it up on the coreboot mailing lists because it's not really coreboot's fault. HP just built a cheap laptop and so it has a junk wifi card.
Hopefully that helps? Cheers!