Frank Castle is one of those 'rorschach test' characters, where the relative merits in any particular incarnation is more a reflection of a given writer's proclivities and the times in which he appears than anything the character really brings to the table.
His original introduction was as an antagonist, and only intended as a one-off b-lister at that. Shallow and two dimensional by design. But the books sold and he apparently got a lot of fanmail they they kept bringing him back, which kind of necessitated making him more of an anti-hero. His early popularity shouldn't come as much of a surprise given this was the era of 'Dirty Harry', 'Death Wish', 'French Connection' and 'The Day of the Jackal' ; bad-cop anti-heroes and assassins abound!
The next big spike from him came out of the 80's Frank Miller era (along with Daredevil and Electra) and well . . . Frank Miller is going to Frank Miller up the joint, and in fairness: at the time his style was a breath of fresh(ish) air for superhero comics. Of course in hindsight (as with most of Miller's oeuvre) it seems the crazed over the top bombast wasn't actually intended to be the parody that it appeared to be, but hindsight is always 20/20.
Then the 90's were a death spiral of 90's silliness (and the bad artwork that typified the period) before Ennis brought the character back from the brink and re-centred him on the whole "veterans with PTSD" angle; as grounded and "realistic" as the character has ever been, and as it happened almost perfectly in time for 9/11 and the massive wave of fetishization of the U.S. military that followed in it's wake. And he's basically stayed like that ever since.
For me personally; I can take or leave Frank. When used well he's a great foil and addition to a balanced ensemble; a great example of a character with an uncompromising "black and white" sense of morality, where the ends always justify the means (without flinching on how utterly unlikable and arrogant such a person would be). Which is why he's such a perfect match for Matt Murdock (AKA: Catholic Guilt Personified.) On his own though, he tends to flounder and wallow. Probably why I never got through the second season on Netflix. by the third or fourth episode I just didn't really care.