I have the older *istD. It has features the DS/DS2/DL don't have, but the newer models have stuff the D doesn't have, so it comes down to what features you want.
Some pluses for Pentax (my opinions):
- uses AA batteries
-great viewfinder
-great ergonomics
-ability to use *any* K mount lens (yes your K1000 lenses will work)
Cons:
-poor to sometimes non-existent support from third party lens makers
-those older manual focus lenses go for stupid (high) prices
-Pentax killed almost the entire line of AF lenses, leading to the point above. They are slooooooooowwwwwwwwwllllllyyyyyyy replacing them with interesting stuff, but slooooooooowwwwwwwwwllllllyyyyyyy
-an uncertain (or at least unclear) camera body strategy. Is Pentax going to concentrate on the low end of the dslr market, or will they step up with pro/serious amateur models? Only the Shadow knows for certain & he ain't talkin'.
I've been happy with the D, but it lacks some things that lesser camera now come standard with (it'll be 2 years old in a month), so I'd like to upgrade, but I'm not sure there's ever going to be a Pentax camera to replace it. If Pentax doesn't produce a replacement D body (or if it lacks features that I want) I'm jumping to Nikon.
If none of that bothers you you'll be very happy with a Pentax dslr.