Well, this is an old thread, but I just watched this one - it's on Netflix now - so thought I'd post.
I was happy with it, but I felt it was a bit condescending and that much of the humor was forced.  If you compare it to the Larry Blamire films, it's much less witty.  I felt the Blamire films were more sensitive too, with a little bit of heart beneath all the humor, and more love for the classics that they spoof.  (I realize Blamire doesn't have a spoof of classic serials.)
Despite the imperfections, Monarch is worth checking out.  It is exciting, and it looks amazing: it really makes the most of the computer effects.  The dark submarines and the moon city look particularly good.
As for serials spoofed, it looks like Spy Smasher and the various Flash Gordons are the main ones.
So, to sum up, I'd say not to expect too much, but worth checking out.
-David