One email that came in about the "Resisting the Urge" article (below) made me repeat something to the responder that I don't remember writing here but is pertinent to the discussion. Here goes:
The two (and really only) appropriate ways to buy something based on need are to buy a "good enough tool" or to buy the "best tool." It should be clear that if you don't buy something that's good enough, you'll be back purchasing again real soon. It's a common problem for many hobbiests, who think that they can just sneak up on "good enough" and end up spending more in the long run than if they had even bought "best tool" in the first place. "Good enough" shoppers need to be especially careful. Sometimes a product sounds like it is good enough but isn't. Sometimes you just think it's probably good enough but isn't. Sometimes someone else talks you into thinking it's good enough, but they were wrong. If you're a "good enough" shopper, you need to do your homework. Carefully and thoroughly.
You'd think that buying between good enough and best tool would be okay, but it really isn't. You paid more than for good enough, yet you still have compromises hidden somewhere in the tool. If you don't find them, you overbought and wasted money. If you do find them, you either mistook where the good enough line really was or you really needed the best tool in the first place and thus underbought and now have to buy again.
Early this year, the good enough line for most people in Nikon DSLRs was at the D5000 (the D3000 didn't make it above the bar, IMHO). The best tool was the D300s, D3s, or D3x, depending upon what you needed the tool for. I'm still trying to figure out the good enough bar today and whether the D3100 gets above it, so I'll punt on that for the moment.
With the compacts I've been writing about (and more reviews are coming), most of us set the bar a little differently. With a compact, my focus bar is different, my pixel quality bar is different, and my noise bar is different. There's also a "good enough" bar for things that I'll carry in my shirt pocket (and the Canon S95 is the best tool there), and a "good enough" bar for things that I'll cary in my jacket pocket. For a jacket pocket, all the mirrorless cameras I wrote about get above my good enough bar, which makes it a bit tougher figuring out which one to actually use, as none really get far beyond that bar. The best tool to date has been the GF-1, but only by a smidgen, and it's still close to the good enough bar.
So my advice is this: if you're a "good enough" buyer, you need to pay very close attention to where you set the bar, why, and whether the product(s) you're considering actually exceed that bar. You cannot impulse buy "good enough." You have to research it. If you're a "best tool" buyer there's little research you need to do. Just define the things you need a tool for and it should be clear what the best tool is. Need to photograph in no light? Get a D3s and some f/1.4 lenses. Period. You can buy on impulse.
Does all this mean that you only need a D5000 if you don't need a D3s? Sort of. There may be specific features or needs that the D5000 doesn't have that might raise your good enough bar higher, but in terms of pixel quantity and quality, you had to go quite a ways higher in the lineup to beat it until the D7000 came along. And not everyone needs a D7000, so, yes, for many people a D5000 is indeed "good enough."
The D7000 murkied the waters vis-a-vis the place of the D300s in the "best tool" category. In terms of pixels (quality and quantity), the D7000 exceeds the D300s. In terms of some features (e.g. video capability), the D7000 exceeds the D300s. But beyond that, the D300s still may be the best tool. Bigger buffer. Better build. Better AF. But like I said, it's murky, because while there are things that the D300s is better at than the D7000 and vice versa, neither scores a knockout punch. Transitions to new technology (in this case sensor) are like that: they sometimes temporarily disrupt the bars. We had that happen before in exactly the same way: the D70 was in many ways (but not all) a better camera than the D100, yet it clearly was targeted lower. The same thing is now true with the D7000 and D300s, but the same thing will happen on the other side: the D300s update should restore order to the bars.