I bought a P51 off Ebay for ~$830 over the summer. It is really well built, and I definitely got a better laptop used than I could have new for the same money.
Great laptop for my needs, 64 GB RAM, came with M2 SSD and took for 2 more drives, and has dedicated graphics card. No complaints.
P50 series was horrible on Linux with the dual graphics setup (the intel + nVidia combo). Just a nightmare for more than 1 external monitor and a docking station setup.
Not particularly pleased with it. Gets really hot (i7 chip) and quite noisy, touchpad glitches out and gets 'sticky' randomly, battery seems to often go to a 'plugged in but not charging state' and the battery life is pretty dire.
Machine itself feels nice though and the performance is good.
I think my next purchase will be an ARM Macbook if I dare to cough up the dough.