All of them. They remain useful until they are completely broken, thrashed, just worn the fuck out dead.
That's why they are doing well. I pay a lot for mine, run them hard, and when they are behind the curve, they get cycled home for various things, until they finally just don't work, and that process is generally painless too.
I like the matte black finish. It's not sexy, but it endures way better than the shiny, "please don't scratch it" finishes on so many machines do. Maybe starting out a little less sexy has it's advantages. Black is damn cool in my book, and there is always that little brightly colored something on the machines, sort of like a great tie on an otherwise boring business suit. Perfect.
The keyboards are a bit noisy, but I like that too. Always have. I can type and type and type until the buttons are all worn, and they just keep going great, no worries.
Heavy little buggers, if you buy the more powerful ones. If I need to clock somebody with my laptop, Lenovo is there! No worries, and I can probably post to Facebook after doing it too.
Linux is well supported across most of the machines. I love that. A Think Pad was the first machine I ran OS X on too. Worked amazingly well, and was faster than the Mac I ended up getting soon after. Gotta admit, the touch pad on the Mac is better tho, but not by much. Some Think Pad touch pads need to be worn in. Once that has happened, they work much better. Weird.
By and large, I leave most of the value added software on the machines. It works well. HP is noisy, Dell just horrible, etc.... I get a competent disc burning kit, defrag tools, etc... Nice package that actually has some real value. On my latest machine they even tossed in the nVidia 3D licenses. Didn't know that, until I connected up to a new TV for some 3D CAD tests. Nice!! That's $14.99 for most of you out there.
Funny thing is I was not a fan early on. One ended up at the house, and I started using it. By the time I got it, the machine was a bit dated, but damn if it wasn't just great to use. When it outlasted some HP thing or other, I was sold. Typically, I get a top machine for work purposes. Need big RAM / CPU, nVidia, etc... Once it's done, it goes home for micro-controller related projects. Long life cycle on these. Worth it.
And... matte finish displays that are typically nice, bright, with fine dot pitches. They've wavered a bit on these on some models as of late. Gotta be a bit picky about that, but so has everybody else. Get the better display they offer, and it's no worry.
The few times I've ordered replacement things under warranty, they shipped 'em, the work wasn't hard. Once the machine ends up at home, I find I can service it much easier than I can the HP machines, which incorporate all manner of fiddly components, glue, buttons that fail, etc... Ugh. Dell sometimes does better, and is in my mind, competitive on this front. Apple? Difficult, but then their stuff works a long time too. Fair game they are playing, but HP is just losing big on this front. Get an HP, and you better hope it works, or service might be very difficult no matter who does it.
I expected some of this to fade when IBM let go to Lenovo. Very pleased to see they've kept the bar high so far. Hoping they continue.