I bought a linux laptop from System76 early this year, and aside from a fiddly trackpad, everything just works. I've seen a lot of people say the same about thinkpads.
Thinkpads have pretty much worked on Linux, for me.
Overall, Linux is great these days if you have compatible hardware. A random Windows laptop might be a problem, but a Thinkpad or a System76 or such should all be fine.
While it worked acceptably for three years, I gave up on my Thinkpad W520 this summer and switched to a Macbook Pro. Partly it was due to size (going from a bulky 15" to a really slim 13" computer is awesome), and partly it was due to wanting application support again, after seven years of almost exclusively using Linux.
One thing I'd like to say when looking at Thinkpads, or other laptops, for running Linux on, don't get one with hybrid graphics. My experience in trying to deal with it was a huge pain.
Maybe my problem was in going for the W-series. The older T-series laptops I've installed and used Linux on were great.
My T42 worked like a dream on Ubuntu 14.04 until recently, and now it refuses to awaken from sleep properly at all because of some update or other. I'm dreading trying to track this down and possibly finding out I'm stuck with using Windows 7.
I had (well, still have, but don't use) a w510. It worked just as well as the t61 before it (which was to say, they both did great under Linux), but it didn't have hybrid graphics... it was Nvidia all the time. The only reason I've replaced it was that it had absolutely abysmal battery life. Old job bought me a comparable ultrabook that weighs 20 pounds less and has 3x the battery life and I couldn't be happier.
I have a T430s - fairly thin with hybrid graphics. You should know that you can switch off hybrid graphics in the BIOS. Please feel free to get a hybrid graphics thinkpad - you can use it whichever way you want.
+1 - Thinkpads work out of the box in my experience, and with a bit of tweaking, I managed to move my parents to Linuxes (Mint) on "random Windows laptops" - a Compaq and a 17" HP.
Same, tried Trisquel and it just worked, except some flash media doesn't play which actually increased my productivity. Coursera videos, youtube work but spending countless hours streaming some TV links won't happen anymore unless I manually install a nonfree flash plugin from repository outside Trisquel, which is enough of a time transactional cost that I haven't bothered
I've got a 17" model. It is plastic, but I have no complaints about the overall build. The keyboard has pretty nice long-travel keys. The trackpad is horrible, the cursor tends to jump around a lot when you try to click. This is my first linux laptop so I don't know whether that's a common driver issue on linux. If not for that I would be 100% happy.
Another model got a lot of complaints about the keyboard a year or two ago, and System76 responded by sending everyone a better one for free.
