This manga is tragically underrated. It takes most of what makes the main Hero Academia story great, and adds a different charm by following a group of street-level vigilantes looking to make the world a better place, rather than the heroes in the limelight. Our main character is one of those lovable everymen, who, though not the brightest guy around, wishes to help people with a charming and earnest innocence that makes him honestly one of the most endearing characters I've encountered in manga or anime. You just can't help but root for him, and every victory he earns is that much sweeter.
It's safe to say that the characters are the strongest part of this manga, much like the original BNHA series. The supporting cast is very different from our hero, but no less nuanced or lovable. They form a very colorful cast that manages to really feel like individuals. Side-characters are also given a great deal of development. It's clear the author puts great love and care into crafting all of them. A villain from one issue will usually become a recurring character and even receive their own small character arc. Before you know what's happening, a character you used to hate has now become one of your favorites! It's truly impressive, and every chapter bleeds its creators' love and care.
But this isn't to say that the story is lacking in the slightest either. What begins as one character's personal vendetta slowly but surely becomes a huge part of the big picture, actually tying very closely into some key aspects of the main BNHA story. It's little wonder the main series has canonized and adapted some of the ideas presented in Illegals! The transition is also smooth and subtle, going from street-level to history-changing at such an easy pace that you hardly even notice until it's already happened. There's one particular turn the plot takes (no spoilers) that ties in clearly into the main series, but it's done so subtly that you might not even notice it until halfway through that arc!
I really can't overstate the case for this manga. If you love BNHA, you will love Illegals. If you're craving a fix after reading the latest chapter, or watching the latest episode, you NEED to pick it up. And if you're already caught up on both ends, well... Welcome to the club.