In addition to the character building rules, the book provides a gazetteer for Star City, a metropolis created by a "space god" in the 1950s off the shore of Connecticut. Star City provides the setting for many (but not all) of the Savage Tales and Plot Point Campaign adventures.
The campaign itself has a nice mix of different types of adventures, as well as a thru-line plot about building up to a massive rebellion and some fun twists and turns along the way. A "mission generator" provides the ability to create adventures on the fly or to link to the various Savage Tales provided.
The idea of playing supervillains has definite appeal for many players; however, there's a set of moral quandaries provided in the course of the campaign that seems intended to push the villains towards redemption. I could easily see this going sideways, with players sticking to their villainous origins throughout - if half the table turns good and the other half remains evil, running the campaign from there could be a major challenge.
My only other concern is theoretical - some of the missions are extremely challenging, and I could easily see high character fatality rates and TPKs from many of them, although I have not playtested them to confirm this. If I'm right, some groups would be turned off by the need to generate new characters on a regular basis.
Overall, I like Necessary Evil's storyline and setting, but I'm not sure I'd run it with either of my groups - the challenge level in particular might be frustrating for my less-experienced table, and the more-experienced one would almost certainly be in PvP mode before the end of the campaign.