Open confession... I like Kai Leng. I don't "like" him. I actually hate him, but the reason I hate him is because I like him.
I think a lot of people see Kai Leng as this failed attempt to create a "cool" cyber-ninja adversary, and he comes across as, ultimately, not actually very cool. He's actually pretty cringe. And while some (maybe many) chalk that up to Bioware failing the character, I think that's pretty much in-line with everything else written about and supporting him.
Kai Leng is a try-hard edgelord assassin who is not nearly as cool or as good as he thinks he is. If he was, he wouldn't have needed all those augmentations in the first place. But, no, he's a petty, vindictive xenophobe who eats other people's cereal ("take that!"), who never wins a single battle in the game without cheating or relying on reinforcements ("take that!"), who Thane openly mocks as an opponent who should be ashamed he was defeated by a terminally sick near-death Drell, who delights in sending you spam messages rubbing your losses in your face ("take that!"), and who ultimately gets a very satisfying and humiliating defeat when he cowardly tries to stab you in the back.
Kai Leng was never a major player in the game, though he tries to be. He's a tool, an extension of The Illusive Man, and he's nowhere near as good or capable as Shepard is and everyone else (including TIM) knows it. I despised him on sight, I hated him from start to finish, and even as nearly straight-Paragon, I will never hesitate to plunge my Omni-Blade into his gut and can't hammer that Renegade Interrupt fast enough.
Sure, he's not compelling or interesting. He never was meant to be. He's just an annoying punk who thought he was big time, who failed upward, and I will never not be happy to send him crashing back down to reality.