I would buy a D70S (yes, an S... not just a 70) over a D60 any day if the week, without hesitation.
I posted a while back... (god I'm quoting myself
giving myself credit for it... how vain can I possibly be?!) (In short, if you have the cash go D80)
In general, the sub-camera bodies are a very different breed from the main bodies (D200/D300). The cameras are significantly lighter, less rugged, and intended as more of a transitional camera. (from point and shoot to more "serious")
They have a variety of features to make automatic shooting of certain situations easier (such as "landscape" and "portrait" automatic modes), and generally are geared towards someone trying to learn the ins and outs of working with a different type of camera than a typical point and shoot. Some of them even come with nice DVDs to explain the ins and outs of photography.
They are also designed to be cheaper (to make them more attractive to a point and shoot owner, as well as make them more accessible to everyone to hook them into this evil habit/hobby), so they cut quite a few corners. As I mentioned they are less rugged, but they also do things like give you a smaller screen, no autofocus motor (limiting lens choices a bit... except for the D80, which
does have this), fewer dials and controls (making it a bit more challenging to work the device when you go into more manual modes), lesser high-iso capabilities, less speed, less software capabilities, etc. For an example, see my thread
here on my review of the D60 from some hands-on experience.
That being said, there isn't really anything
wrong with the cameras. You can get just as beautiful shots... it just may take a bit more work and you may find that someone with a "more serious" cam might be able to outdo you here and there. Honestly, very rarely, but they
do have a better tool, so they're going to be a bit less limited. You have to keep this in perspective, though... I think my D100 is an amazing camera, and in almost every way, all of these newer lesser cameras kick the snot out of my D100... the only real advantage my poor D100 has is that it is more rugged and can take the vertical grip. (I don't believe most of these cameras can... not sure.)
Now,
that being said... the D80 is really exceptional in my opinion. I recommend it to people all the time. It's really just a smallish step down from the D200. It's still clearly less of a serious camera than the D200, but the differences are no where
near as significant as they are on any other camera in the sub-body line. It has a bit more capability, doesn't treat you like you're clueless, but still has the capabilities and features to help ease the transition if you need or want them.
In summary on the sub-bodies... The D80 is a
great camera. The other ones, in my opinion, are all kind of frustrating. They're fine... but frustrating. I would almost sooner buy a used D70S than a D40, and possibly even a D60. The D60 really annoyed me. Unless you are scrounging for cash to make this happen, if you decide to go with a sub-body, get a D80.