I bought a D5000 this week at Costco. It was meant to replace our D40 as a family / throw in the backpack kind of camera. I was looking forward to being able to shoot some video of our kids, as well as having the newer features like CA control, distortion control, ADL, etc.
Well, after comparing the pictures side by side to the D40, using the 18-55 kit lens on both, I returned the D5000. I was a bit sad since I really liked the D5000 and it's features, but in the end the decision was very clear. At base ISO 200, the D5000 files have more noise in the shadows in practically every scene. Also, there is hardly any extra resolution, despite the sensor being 12MP vs. 6MP. With the kit lens anyway. Yes when I mounted one of my fancier lenses, I could take advantage of the extra resolution, but that is not how I was going to use it. For serious pictures, I still take my D700. Also, I prefer the colors I get from the D40. I was able to tweak the picture control on the D5000 to get close but the D40 is still much more consistent. And there were situations when the D5000 AWB was way off, putting a green cast onto a scene, while the D40 delivered the same scene in natural colors. And finally, the videos from the D5000 are not all that great. I get better video from my 4 year old Sony P+S.
All this does not mean that the D5000 is a bad camera, far from it. But it is just not a worthy upgrade for me from our D40, and I would not even do a straight swap for it. It just goes to show how good the D40 is. It would have been great if Nikon had kept the 6MP sensor, perhaps improved it even further, and just added the extra features.