I don't have any direct experience with the Eneloop adapters, but Sanyo is rolling out true C and D sized Eneloops starting in Japan next month. You can read about it
here . No idea when they will be selling them in the US. Oh, and a new charger that will accomodate all Eneloop sizes.
I have used adapters in the past and they do work but you--obviously--have much shorter runtime that you would with a true C or D-sized battery. The fit can be problematic depending on the capacity of the cells. Unlike alkaline AA's, which seem to all be pretty much the same size, higher capacity NiMH cells tend to be a bit fatter and may not work in adapters designed for a lower capacity cell so they may or may not work with your cells.
Edit: Wait, not sure why I'm fixated on Eneloops. Low self-discharge C and D cells are already available from Accupower, too.