I was way above the target audience when I started the original series (I was on a Rick Riordan kick) and (realizing that I am out of the target audience) I always thought it was an okay series, kind of educational, problem-solving, quest-type series with likeable protagonists for juvenile readers. Then the second series started and against my better judgement, I kept reading ... after the first couple C vs. V books, I was over it. Frankly, I've only continued with the series to see what happened to my favorite characters (Amy, Dan, and Ian). 
 The finale --on one hand, as other reviewers have noted, it seemed a bit rushed, loose ends quickly tied. However, other aspects, such as the whole hostages plot, had felt like it had been drawn out for too many novels ... 
 This entire Cahills vs. Vespers series really highlighted how inconsistent characterization can be when having multiple authors writing a series. It was tolerable the first time in the original 39 Clues, now it is simply too much for me to overlook. In my opinion, everything about this Scholastic series, from featuring "famous" authors: Rick Riordan, Jude Watson, Linda Sue Park, David Baldacci, etc. to the trading cards ... it seems like it's all about the hype or just marketing to make this a popular series, not because of a good plot or quality writing (I've read work by many of these authors and they are so much better writers than this), which is very disappointing as a reader. Now with a rumored third series to be published, I have yet to decide if I will continue this series, or just cut my losses now.