Would someone who has used Miller Solo DV aluminum sticks describe what the leg locks are like. Do they twist until firmly secure or is there an actual click similar to Gitzo's G-Lock. I know the locks are described as concentric but I am not familiar with how a concentric lock works.

Twist until secure. There is no firm click, despite that, i've yet to have an issue in 3 years of using mine.

I have a couple of fairly new Gitzo tripods, and the collars on the legs don't click to lock. The collar just twists and tightens.

I believe that G-Lock refers to the fact that downward pressure while the tripod is in use tightens the the lock. ("G" standing for "gravity")

Also, the leg doesn't twist when the collar is tightening, making it easier than older models to work with. I know some other brands have this feature now too, while some do not.