I've owned the DS bodies for a year and a half, also own several other cameras with custom-form-factor Li-Ion batteries. Frankly, the other cameras are easier to manage. No fussing with two or four cells, no self discharge, smaller, lighter cells, etc.
The DS body I use for tripod/macro work has had a set of CRV3 Lithiums in it for 7 months and somewhere around 900 exposures. They cost me $20, but will likely run another 300 shots.
But the battery king in my cabinet right now is my Sony R1. I charged the Info-Lithium battery three months ago and have made 500-600 exposures with it, and it still shows 140 minutes time available in record mode (330 minutes when fully charged...). That's with an after-market, 1400mah replacement for the Sony FM50NP (1150mah) that cost me $24. At this rate, I'll have to charge it twice a year. Since it's rechargeable, it ends up costing less than the Pentax on battery costs.
Godfrey