Seidio has 3 batteries on their site for the Droid
Extended
Slim
Slim Extended
The Slim Extended with more power than the OEM is still under development and the capacity is TBD and the one linked on the OP is not described as having more power in the same form factor.
So basically the current slim battery they sell is just meant to be a regular spare(although it is slightly less powerful than the OEM), right now it's on sale for $25 with code DROIDBATT if anyone is looking for a spare.
I'm holding out til they release info in capacity of the Slim Extended before I decide what my extra juice option will be.
I had the Seidio extended on my Curve, but was not a fan of the extra bulk, don't think I want to do that again, the Droid won't look hot with a big butt! The external charger would be handy too, I may keep an eye on the price and pick one up if I see it cheap enough.