I've been using different models from the X-Series with Ubuntu since 08.04 and am currently looking at the T450s (14", but still an Ultrabook and light enough for me).
The X1 Carbon (latest.. 4th? gen) is my daily laptop since release and I can't wait to get rid of it. In anything other than low light/perfect lighting the screen is unbearably dull, speakers are appalling (though in ubuntu you can override to > 100% volume). Keyboard is pretty unsatisfying to use also.
I got it because i wanted a 14" laptop and it was the only one on the market. Screen size is perfect (15" is a bit big for travelling & carry on luggage)
I was surprised how light the T-Series models have become, hadn't looked at them in years and only remembered the heavy clunky units that made me go for the X-Series.
The current model T460s is also interesting, quite a bit more performance than its predecessor, albeit in a different price bracket.
I'm mostly going for the 450 because there's tons of decent refurb available and I'm frugal that way ;-)
Just be sure that you're comfortable with the trackpoint on the 450... They swapped the buttons for a touch sensitive area at the top of the trackpad. The physical buttons have made a welcome return on the T460 range, but if that doesn't bother you, enjoy a great machine.
Same here, except I have a different experience. Maybe it's because I upgraded from a 2010 Macbook Air, but I like the screen a lot. The speakers suck, I agree, but I rarely use external speakers when I'm in laptop mode. I really like that it runs Ubuntu almost perfectly, and that it has 16GB or RAM. I do wish the video card was more powerful, but it drives a 4k monitor (as well as its own display) quite well. It's stupid lightweight, too. Makes my 2010 Air feel like cast iron.
I've been using different models from the X-Series with Ubuntu since 08.04 and am currently looking at the T450s (14", but still an Ultrabook and light enough for me).
The X1 Carbon looks damn sweet as well.