Keep in mind the D cell numbers are very misleading IMO. While D cells have a large amount of capacity, they provide much of their power at very low voltage.

Typically this means you will not get anywhere near the rated capacity from the batteries because 1) you'll replace the batteries well before they are dead or 2) the device will stop working because it requires more voltage than the cells will provide.

The main exceptions to this are applications which do not have large power requirements. Consider for example, running a handful of 5mm LEDs from 3xD cells. Darn thing would probably run pretty much forever.

Also, the D cell strobe lights probably make good use of the D cell because they charge a capacitor which stores up prior to discharge. As the voltage drops, it might take longer to charge up the capacitor, but the light will still be bright and it will probably run for a very long time.

Some very good information in this Alakaline Battery Shootout by SilverFox on CPF.

All in all, I'm not very fond of D cells. I think in most of the cases you'd be better off with 3 to 4 AA cells instead.

-john