The second Chronicles of the Lensmen is much meatier (for the lack of a better term) than the first volume. All the groundwork has already been established so Kim Kinnison, the Gray Lensman, gets right down to business.
One thing that stands out about the Lensman books is that it's clear Doc Smith had things well-planned when he started. While the repeated revelation that the supposed big bad isn't at the top of Boskone could have gotten tiresome, it feels rather natural. I also like the introduction of the Black Lensmen. Once again, pay up Green Lantern! Sinestro is a direct analog of the Black Lensmen.
The Lensmen books are actually fairly relevant in today's world. The Galactic Patrol is a lot like the United States and the Boskone, especially in the later stories, are terrorists who strike without warning. I'm sure Smith meant for them to represent the communist threat of the 50's but it still works today.
The end of Children of the Lens was as I expected. The Kinnison children fulfilled their destinies. I've heard that Smith had another story in his head featuring them but it was unpublishable due to the controversial nature. If you've read CotL, you have a pretty good idea of the story's next logical step.