Have/have had the following Thinkpads: 600E, 600X, T21, T40, T60, X60T, T61, R61, X61T, W500, X201, X220, X230, Edge 14, and SL400
I don't like the direction Lenovo's taking with the Thinkpad. The change from the traditional 7-row keyboard to the chiclet-style keyboard, the disappearance of the dedicated mouse buttons, BIOSs that are more heavily locked down (whitelists), and the more cheesy body design are my biggest gripes with the current generation. Hell, the keyboard change itself made me revert to my X220 for daily use. I've noticed a very discernable decrease in build quality from the *61 line and on (coincidentally the first line to be produced since the Lenovo acquisition). Plastic bits breaking off and problems with the screen are the issues I've experienced most.
In addition, what's with giving the X240 only single-channel memory support and the same crappy 1366x768 resolution?
Still, they're incredibly sturdy and very easily serviceable. I've replaced screens, motherboards, CPUs, pretty much every removable component there is in a laptop and I've never had an easier time working in a laptop than with the Thinkpads. Hell, IBM/Lenovo put out complete manuals for each Thinkpad telling you how to take the thing apart and put it back together.
Customer service is also top-notch. I've always gone through the Atlanta service base and they've been nothing but helpful when it came to fixing my crap under warranty. If it's a customer-replaceable unit, they'll two-day air you the part without requiring a credit card; they don't even care if they get the broken part back or not. The times I've needed to send my laptop in, everything was shipped next-day air and my laptop was fixed the day it arrived at the service depot.
So, while the physical aspects of the Thinkpad line have me willing to open up to more manufacturers, it's probably the intangible qualities that'll keep me coming back to them.