I only have an older 18V battery Dewalt at the school and the 20V MAX.
At one point, DeWalt released a 36V MAX version that was based around the A123 system's advanced LiIon battery. I actually used 10 of these as part of a Formula SAE Hybrid car ~5 years ago. At the time, the big thing was they had an extremely high discharge and recharge capacity versus their amp-hour ratings (~10C as I recall).
I haven't researched it fully, but I think the 20V MAX is essentially the same battery technology. I know A123 went bankrupt a couple years back... but I get the impression that the nice thing about the 20V MAX is it is very powerful for the size or alternatively very light for the power... and that it might be a similar battery technology.
It all depends on how you use it whether it is worth the money. For all the home projects where I was screwing drywall into a ceiling, or any project where I'm working overhead with a drill... every pound I cut down was very much appreciated. For general robot or shop work, the little extra weight from an 18V might be fine. If you are using the drill how it is supposed to be used and not abusing it.. the 18V might be fine.
As I said, I can't say that Dewalt is better than Milwaukee/Makita/Ridgid etc... just that for $99... I sure love mine
