Weirdly published after Jupiter's Legacy but cast as a prequel, having read them in 'chronological' order I would definitely recommend it. The two series seem to be releasing alongside one another, which is a shame because the world of Legacy felt so much more vibrant as a result of having some familiarity with the characters. Without Circle a lot of Legacy's impact would actually feel more derivative.
That said, Circle is a brilliant introduction to this universe's first superheroes, even if it leaves the actual origin story up to the bigger brother. Rather than being its own specific plot, Circle is clearly being used to flesh out the world and characters within it, consisting of several "one-shot" style short stories. Each focuses on a different hero and stands fairly apart from the rest of the bunch, often feeling differently paced or toned, although when read in order an undercurrent becomes apparent. It's a nice way of providing a wide range of stories whilst still resulting in a promising finale.
The tales themselves also have a very broad scope, though the focus is much more on the person than the hero. By that I mean, whilst each story is ostensibly superhero fare, with villains, monsters and team-ups, the focus is actually on the human interactions taking place around the 'comic-book' plot. You see how the heroes interact with everyday civilians far more than they beat up bad guys, whilst getting a very personal look at their flaws rather than their strengths. In fact, what powers each hero has is largely left unmentioned. Some are fairly obvious, such as flight, super-speed or super-strength, but once again these really only become known to the reader as asides rather than focal points.
It's a very refreshing change, accepting that the reader likely knows how superhero comics and novels work and won't benefit that much from a detailed analysis of that kind of stuff. It's a mature look at how prevalent the industry has become and, I think, completely fair, stripping away the hand-holding of the Golden and Silver Age stories that Circle so clearly homages. Just as the hero elements are downplayed, the personal ones have a light shone on them, especially through the unique historical lens the series has. Being set in the mid 20th century the heroes expose the dark side of those comics they clearly mimic. This is perhaps clearest in one of the better stories, looking at how a superhero that fights for morality can cope with being gay during a time when this was seen as immoral. It's a heavy story with some very adult and hard-hitting themes and consequences, and a thoroughly fascinating read.