This product is, at its price, pretty much indispensible for anyone who wants to run a paragon tier campaign the way D&D was always meant to be (with followers and holdings, of course, the traditional name level activity!), or a heroic tier campaign that finds itself wandering into "war, war never changes" territory early on. It can even be appropriate for the grand finale of your epic tier campaign. With the crunch on 4th edition with the announcement of Next, of course, the price is a double bonus - you don't have to invest a lot to get the quick fix you need.
The concept is one we've probably all thought about before: one mini or token now represents a lot of guys instead of just the one. But what this supplement offers is more than just "play Warhammer or pretend your miniatures are each 100 orcs", it gives thoughtful options for different tones (heroes fighting dozens all on their own, minion style, or having to give their own armies a general-style attachment boost) and even the way to structure fantasy campaign season to provide realistic backdrops for all sorts of roleplaying opportunities. I wish it gave just a little bit more, but squeezing blood from a rock like "1 guy = 100 guys!" is hard, and One Bad Egg has done admirably on that front already.