If you are looking to create truly unique outdoor shots, sooner or later there will be a need for a large light source, with no where to plug a studio light. At just under $300, this is a very economical solution, we used this the other night on the top of a mountain overlooking Provo. It was the first time I had a chance to play with one and for the most part it provided all the power and more for the hour of the shoot. (And we had the model light on the whole time between flashes).
Here is my take on this: If you do a lot of outdoor shooting and have a very specific need for large light sources (lightboxes, or shooting with the sun as a back light), get it. There is no other way to get amazing studio light exposures shooting into the sun, in pitch darkness. This is really for powering the studio lights in the middle of you know where. (Other power packs run around $1500-$2500).
If you are creative enough, and are good with a few off camera flashes, you can probably make due without.
Vagabond Portable Power Pack