Some TV shows hit the airwaves and have an immediate impact. There's no need to find an audience or for the actors to settle into their characters, it's full-throttle from the series premier.
Miami Vice is one of those shows. The look, the scene, the cars and above all the music made it immediately compelling to watch. Sure, there were some rough edges in the first season, particularly in the way they loaded down Don Johnson's character with a ton of quirks. His Sonny Crockett (alias Barnett) not only has to balance work with raising his son and save a troubled marriage, he is also a former Florida State football player and a Vietnam veteran and he lives part-time on a sailboat with a pet alligator named Elvis. Almost immediately writers decide to forget the football thing, Vietnam will fade and his wife will divorce and move away. Elvis also fades into obscurity within a couple of seasons.
As a writer, I get why they loaded him up. By 1985, the market was saturated with cop and detective shows. The format allows for lots of guest stars, but mostly it boils down to "solve the case of the week," and so the only way to achieve separation is to have quirky characters.
Turns out, Miami Vice didn't need that - at least not at first. While the format was familiar, the setting and approach was unprecedented. Michael Mann didn't just mix in some new elements, he created a unified aesthetic that combined color, music and aesthetic in an unprecedented way. Miami Vice wasn't a show so much as a mood. He also balanced Johnson's quirk-laden Crockett with Philip Michael Thomas' more traditional Rico Tubbs, a New York detective who has a more old-school approach. Johnson set a fashion trend by combining Armani suits with pastel t-shirts and stubble. His weapon of choice was a Bren Ten carried with spare magazines in a shoulder holster - about the most firepower one can feasibly conceal. Thomas was more traditionally dapper and his weapon was the tried and true snub-nosed revolver (sometimes backed up with a short-barreled shotgun).
Their partnership was iconic and within a few episodes, they're a solid team. The supporting cast is also excellent, particularly after Edward James Olmos joins the team four episodes into its run. His Lt. Castillo is quiet and stoic, the perfect foil to the flash and energy Crockett and Tubbs (and he naturally uses a traditional patrolman's revolver).
Of course, no discussion of Miami Vice is complete with looking at the peerless music that was incorporated into the show. Jan Hammer provided both the main title theme as well as incidental music during the show itself. In addition to Hammer, Miami Vice featured some of the hottest musical acts of the time and incorporated the sound and lyrics with what was happening on screen. It was like an hour-long music video.
Actually, it was more than that. Especially in the early seasons, musicians actually appeared as characters in the show. Episode 16 featured Glenn Frey as a drug smuggling pilot. The episode was titled "Smuggler's Blues" and featured his song of the same name, which was the leitmotif of the episode.
This was typical of the early seasons, and Phil Collins, Frank Zappa and countless other singers got roles that also featured their work.
Everyone who was anyone got involved. Lee Iacocca and G. Gordon Liddy got prominent parts.
Thus, the show started out strong, refined itself to become even stronger, and then inevitably began to decline.
The first step was the departure of Michael Mann. Without him as producer, his vision inevitably was compromised. He had strict rules about the color palette of the show that were no longer enforced. Even so, the show did immediately decline.
I'd place the hinge point after the third season. By that point the formula was starting to get stale and even the cast was losing interest. John Diehl, who played a supporting role as Larry Zito, got so bored that he had his character killed off mid-season and no one replaced him. Diehl and Michael Talbott (who play his partner Stan Switek), had gotten some interesting (and funny) sub-plots in the first two seasons but the show was starting to take itself seriously, and they were relegated to the background. It probably did not help matters that the female cast (Saundra Santiago and Olivia Brown) got much better treatment by the writers, often having entire episodes built around their characters.
It was really the fourth season where things started to go downhill. By this point the writers were starting to recycle material and resorting to gimmicky celebrity appearances (such as Sheena Easton doing five episodes as Crockett's romantic interest).
The romance angle was particularly troublesome. No one expects the show to shift into a relationship drama, but there were better ways of handing some of the romantic plots for the various characters. For example, and obvious one would be a recurring role as an old flame in an on-again, off-again relationship. Instead, the writers mostly kill off the detectives' girlfriends, which quickly loses its shock value and becomes yet another trope.
Probably the final straw was the cliffhanger at the end of season 4 where Crockett suffers a traumatic brain injury and becomes his drug-dealing alter ego. It's an interesting take on the usual amnesia plot, but it's also a desperation move, and when the three-episode arc ended, the show seemed creatively exhausted. Even the music was of lower quality.
One of the only bright spots of the last two seasons was Martin Ferrero's portrayal of Izzy Moreno, a street informant who always manages to steal whatever scene his is in. Ferrero was involved from the start of the show, and turns up whenever information is needed, but is always involved in some con. As the show deteriorated, these became more amusing, but also more improbable. Miami Vice was alternately grim or silly, which is impossible to sustain.
As a viewer, I'd stick to the first three seasons. There's no real story arc, so the best approach is to enjoy the ride at its wildest and then get off before things get boring.