The 5510 is a lot more rigid than the 3800 was, but it still isn't rigid enough. I could carry my personal 15" macbook pro all day with one hand while still being able to click the trackpad down. With that 5510, the chassis flexes too much for the trackpad to click when you hold it from one side. And don't get me started about the cheesy chargers Dell uses.
The 5510 was worlds better, I'll admit - and it does a decent enough job running Ubuntu! There's enough to like about the laptop, but not necessarily at the price they charge.
That said- I'm staying with the m3800 as long as I can. It's a few years old and my battery needs replacing, but otherwise is very capable. Some people have had docking issues on and off with the 5510 (occasional crashing, though I think it's a driver/software issue), but more importantly it has the awful "Nostril Cam" which is annoying for video meetings.
In general, fast, light, good battery (for perspective, mine lasts 2-3 hours now, vs 5-7 when I got it), useful ports. The keyboard is not the best (home/end/pgup/pgdn keys are fn+arrow instead of dedicated) but very usable. I hated the glossy screen at first, but touchscreen is surprisingly useful and I got past it. I just keep a microfiber cloth in my laptop bag now.
At least the Precision line is the buisness line which means better support, preinstalled with Linux (so its tested well) and probably from what I have read better tested/built due to being a business line. I like supporting Dell on this venture b/c they are sticking thier neck out a little by even supporting/preinstalling Linux as a big name OEM provider.
The Lenovos are great too, but I wish they would officially support Linux and offer Linux preinstalled as an option so you don't have to pay for Windows.