Dendrobates is pure distilled action movie badassery in manga form, without being too oversexed a la James Bond. A real treat for people like me who love action flicks but find the Hollywood forumula, where the (white) good guy fights black/Latino/effeminate/Muslim, etc. villains and emerges victorious with the beautiful (also white, or perhaps Latino) girl in his arms daunting.
Clean, dynamic art. An ultra-confident, charismatic, super-skilled, ruthless - and yet somehow chivalrous - protagonist. A cute love interest who offers the MC a link to retaining some form of empathy and emotion while (thankfully!) not being a source of out-of-character angst and a desire for "normalcy" that drags him down. An enjoyable supporting cast who play their roles well but don't detract too much from the goal of the plot, which is basically to have fun watching our near-infallible protagonist shoot drug lords, gangbangers, murderers, and others of that ilk in the face. All tried-and-true action movie tropes, manga-style.
I'm always puzzled when people complain about "lack of character development" in stories like this. The core cast is composed entirely of adults above 25 or so: adults are pretty set in their ways and their views of the world, have stable careers and a routine they've perfected over years. They aren't going to change their personalities, opinions and life goals every other day like a bunch of teenagers! And if you think the villains "aren't sympathetic enough," that's just silly. The antagonists in this series are yakuza/mafia, rapists, child molesters, mass-murderers, thrill-seekers who harass and torture and kill because they're bored, people who destroy entire families for a few thousand bucks. Portraying such characters as sympathetic is utter nonsense in my eyes, because I really don't buy into fiction's current trend of "grey-and-grey morality." This world isn't grey-and-grey, it's more like grey-and-black, and our (very dark) "grey" male lead murders his very "black" opponents, for a fee. If that sounds problematic to you, you will not enjoy Dendrobates.
Another comment mentioned that there's no indication Sengawa feels any remorse regarding his actions. The short answer to that is: he doesn't feel remorse. Sengawa was raised as an assassin by a South-American underworld boss, vowing to one day seek vengeance for his murdered family and friends: I doubt "repentance" is even a word in his vocabulary anymore. He seems content use the skills he gave up his childhood to perfect in order to wipe out as many underworld figures as possible, and doesn't waste time navel-gazing. Personally, I find it extremely refreshing to see a non-villainous main character who's so unrepentant and uninhibited.
On a less fangirlish, more down-to-earth note: I like how the mangaka doesn't try to pretend that Sengawa's actions are going to eliminate crime in Tokyo, or even reduce it. He makes it very clear that the man is not an angel of justice, but an avenger; and an adage of sorts that's repeated throughout the manga is: "When one crime lord falls, another will rise to take his place. [But that doesn't mean we stop fighting crime]"
Tl; dr: Don't expect a lot of depth or philosophy from this manga. Read it when/if you're in the mood for a fun action story with a cool, good-looking, nigh-invincible lead. Nothing more, nothing less.