In a fit of nostalgia, I'm re-reading my way through this series.  I've always loved Encyclopedia Brown, and I remember back 30 or 35 years ago I was  always running to the library to see if they had any new ones.  They're fast, they're entertaining and very well-written.  It's so easy to devour one in a half-hour and yet you're just as satisfied as if you read a full length novel. 
 
So, this review will sum up the series for me:  great for kids, and even better for adults.