I was really looking forward to this Handbook and it fully delivers as a resource book on how to run a horror-based campaign, but more in an Agent of Freedom sort of way than as a supplement for superhero teams.
The background material on how to run supernatural adventures is very good and it comprehensively details the various horror options, stereotypes and backgrounds you can use. The problem is that it feels almost like a background for a separate campaign to Mutants and Masterminds rather than something that can be added to an existing campaign. There are numerous guidelines on how to run the various types of horror adventures for the various powerlevels but the over all feel of the material was that this is primarily aimed at skilled normals or lower level supernaturally-powered characters than level 10 and above powered metahuman heroes.
The various graphics are nice but those used to illustrate the character stereotypes in particular turned me off as they were more Sandman in style than Superhero and didn't work for me personally and the included organisation, Arcade is too US-centric for those of us who are not running adventures set in America (and would be extremely difficult to transplant to say UK or European soil) and again fails to provide sufficient hooks for including superheroes except as occasional support to Arcade's existing investigative teams...
Overall this would be an excellent supplement for streetlevel heroes (hence the 4 stars) but would be harder to make work for higher power levels and at present I'm not sure how useful this will be in an existing campaign - even one that is already magic-based.