Weird indeed, I think there's really two categories of "external batteries" out there for two different kind of users. I will classify them as such:

sub-$100 external batteries adapters: provide a few hours of battery lifetime, small, light, cheap, disposable.

$100 and above ( such as the ones found there: www.batterygeek.net ): professional use, heavy, expensive (or sometimes VERY expensive) will power any device, reliable.

Depending in which category you belong you may have to break the piggy bank to do serious work... or not. I personally see it as an investment, such as this is a battery you will keep for 3+ years, while you will change PDAs/Laptops/Cameras a few times between. May be worth it, I'm tempted getting one, we'll see how many days I can keep the N810 up; that could be interesting.