There is one thing that really worked about Kai Leng though: On my first playthrough of ME3, after Thessia, I really REALLY wanted to kill that bastard. I am usually a very calm person and don't hold grudges, even against the worst kind (I let Balak live and recruit him in ME3, etc.) but this guy, I was really looking forward to finally take him down. That because he so obnoxious and because he wins in an unfair way a couple of times.
Don't get me wrong, I think there are a lot of problems with the writing, and presentation of the character and if you read Retribution, his game appearance is abysmal. In terms of the gameplay design decision in your encounters with him, things are even worse and we've done much complaining about this already (and rightly so). But they did really get me - the player - riled up over this guy and the final scene with the omniblade was really satisfying as well.
So while there is more than enough to complain about Kai Leng, I guess ultimately, they did achieve their goal with the guy, at least as far as I am concerned.
Clone Shep on the other hand is just fun. As Iakus said, it works because it's not meant to be taken seriously.