It's a direct continuation of Ennis's 12-part 2000 series, Welcome Back, Frank, and it ran until Ennis rethought what he wanted to do with the character and moved him into Marvel's adult readers imprint, MAX. In other words, this is the lighter, funnier version. It's a laugh, although a little more diluted than Welcome Back, Frank. It's still insanely violent, mind you. Frank uses guns, guns, a nuke, guns, a steamroller and sarin nerve gas. A journalist meets a wood chipper. The dentist torture episode is a riot, while I howled at Joan the Mouse reluctantly assisting a bedridden Frank during a kill. The fundamental joke with the Punisher is that he's a humourless block of wood, on a psychotic mission to kill in every way he can think of. If that doesn't sound funny to you... well, you should either read more or less Garth Ennis.
It's not a superhero comic. It's basically a 1970s Wagner/Mills British adventure comic (2000 AD, Battle, etc.) that's infiltrated 21st century Marvel. That's what Garth Ennis grew up on. He's not interested in superheroes. He likes ultra-violent bastards who don't recognise morality, justice and appropriate behaviour. The Marvel universe hardly even exists in this book, with the Punisher's enemies usually being gangsters, drug runners, ex-military, etc. Occasionally, we meet heroes like Daredevil, Spider-Man and Wolverine. Crossovers help sales, after all, but Ennis mostly uses them for hero-bashing comedy. His superhero crossover story is called "Confederacy of Dunces", for instance.
(a) Daredevil (and Elektra) he genuinely likes and they get off lightly in the comedy punishment department. (Daredevil's a top lawyer, so he probably is indeed the most intelligent of that trio I mentioned.)
(b) Wolverine becomes an idiot and a joke. I like the character, but this was brilliant and well past time. What's great about 21st century Wolverine is that his healing power makes him indestructible... so Frank can do anything to him, no matter how ultra-violent or gross.
(c) And then there's Spider-Man. He's not a Wolverine-level butt monkey, but Frank's (ab)use of him in issue 1 made me laugh.
Mind you, the Punisher knows that any real superhero could flatten him in seconds. He has no superpowers. If you've ever read The Good Old Boys, Ennis's Preacher special with Jody and T.C., that's not a million miles away from the Punisher.
Is this series great? No. It's light and silly compared with Punisher MAX, but it's also less funny than Welcome Back, Frank. That was shorter and more concentrated. This Marvel Knights series is good fun, but it has weaker story arcs (e.g. the cops) and it doesn't have much of an ending. Preacher also faded away towards the end. (Mind you, strictly speaking I should be calling the end of Marvel Knights a transition, not an end.) Only occasionally does Ennis talk here about something meaningful, e.g. what it means to be a cop in modern America, or liberal discussion of killing homeless people, or racism and homophobia in Texas. (The latter story's also a Western.) These stories aren't bad, but the strongest of them is the one where the Punisher visits Belfast and Ennis lets rip about the state of his homeland.
There's nothing wrong with not being deep and important, though. It's lots of fun. I ate it up with a spoon and I continued by buying all of Punisher MAX too. (Incidentally, I bought the Marvel Knights trade paperbacks that omit the brief non-Ennis run in issues 8-12, which tells you all you need to know about how Ennis's Punisher work is regarded compared with other people's.) The stories are offensive, of course. (The dwarf massacre, for instance.) Ennis wears his opinions on his sleeve for comedy, e.g. the French. He's proud to be vulgar. Someone gets his head urinated on. "Something important fell out of his bottom."
Occasionally, it's disturbing. The police and their support people are worryingly quick to consider suicide, for instance. But a Punisher series probably should be disturbing sometimes.