Is a used ThinkPad T440/440s (or T430) worth it right now? Assume that it's powerful enough to run whatever I need, and weight isn't really a concern because it's going to spend most of the time sitting on a desk or in the library where the extended battery will come in handy.

I'm eyeing it because build quality and keyboard, and because I can get one for a pretty good price (if I buy a new laptop at the same price I'll get a shitty celeron or low end i3 based thing)

Edit:
Cut the core i3, wouldn't be able to get one at that price

I'd expect the battery life to be much better if you buy a new cheap one. Check some benchmarks to see how the actual cpus compare.

Well, I have got a thinkpad l540 that is at least 4 years old and very used, and it runs very well... but not for everything
I used it with Windows 10 recently and it lags a lot, due to the hdd. I then added a ssd (30$) and switched back to pop_os!, and now it is very fast, practical for compiling, debugging... Especially the keyboard is really good

As this is a similar computer, i think it could be the same. But, with an HD screen (i have full HD) and hdd but no sdd, i don't think it can be agreable to use everytime.
So make your choice, but don't spend spend too much money on it : for more than 120€, I don't recommend