Nobody here that has bitten by the 'iron bug' is gonna stop lifting for a month just to do circuit training.
Now, if you are going to test for the academy (fire, police, etc.,) it may be a viable choice as you mention "conditioning, endurance, and functional strength"
Functional strength alludes toward having somewhat of a "cross-over' effect from the gym to your daily life. Excess muscle is not geared toward making your life easier, it's about appearance.
With all of the questions that you seem to have here lately, I would think your best interest would be choosing one objective, and sticking with it.
What is it that you want? More muscle? Better conditioning? Lower bodyfat? Functional Strength? Cardiovascular endurance? All of the above?
You can't have it... easily.
Circuit training, especially at high speeds and high heart rates is going to crossover into cardiovascular endurance. The first thing to go bye-bye in a 'natural' athlete is muscle tissue. So why work so hard to gain maximum muscle only to sacrifice it to 'endurance' training?
In my estimation, it is a waste of time, unless of course, you have a sport-specific (or job-specific) goal in mind.
Choose a basic weight training program and stick with it. If you want to add cardio before or after each session, so be it. Whatever you decide-not only will your training program be key, but so will your nutritional program.
I would assume that you want size. Size will not come overnight. You will lose it easier than you will gain it, so beware during your functional, cardiovascular endurance training.