One of the talkers on Mac Break Weekly (#145 06-16-09) swore by the Air as his de facto traveling companion.
On the other hand, I've seen some posts that seem to imply that some battery packs may not be "compatible", to the same degree that some charger cables might cause the 3G(S) to complain about a non-compatible accessory.
Here's a YouTube unboxing review by
. He has other reviews (IIRC) comparing it to the Incase Power Slider.
YMMV