I'm using a *istD and quite like it myself. Never really used D70 or 300D before, although tried them in stores. *istD is a bit smaller and lighter, *istDs is even smaller and lighter
Base on all the reviews I read, all these DSLR cameras are good enough for us to use. Each has its own pros and cons. You won't regret with any of these three.
But one thing you have to consider: LENSES. Here in SG it's really hard to get Pentax lenses, new or used. If you don't have an SLR camera before, I suggest you choose either Nikon or Canon. They are equally good, and in same price range too. Try them in a camera shop and see which one you prefer. If you have an SLR camera, then the choice is much easier .
Reason for not much comment is basically due to the strong Marketing of Nikon and Cannon. Pentax, very weak in this areas.
I am using *istDS, I think one of the early few who got this camera @ Sitek 04... I have played around with 20D, 300D and 10D, and have considered D70, but drop the idea due to the weight. Picture quality wise, I quite like all of them, but I still prefer Pentax..
2. SD card (widely and commonly used)
3. Use AA battery..(easy to obtain even in overseas)
4. Picture are more natural
5. More solid feel, unlike 300D.
6. U feel special, cause it not common..
1. Lens not easily available, even 2nd Hand. ;(