I owned a 600m for about 2 days. The keyboard flex was very tolerable in my opinion - it felt like most other laptop keyboards that I've used. Yes, it did flex a little if you REALLY pushed on it, but who needs to put 400 N of force on a keyboard while typing?
As for the build quality, it was rather flimsy. The whole "it felt like it was going to fall apart when I picked it up" might be a little pushing it, but it did have a very plasticky feeling. The LCD was easy to distort, though it went away immediately after pressing on the back of the screen. My main concern was how easy it was to damage my particular 600m during shipping (no damage to box, no damage/scratches on LCD and yet LCD was screwed up? Wow.. easily dislodgeable video flex cable anyone?)
If you want construction quality, check out IBMs or Toshibas. The latitudes seem better to me, but I have no personal up close experience with the new D series.