I reviewed the whole series on my blog a few years back (link in my signature), though they're spoiler-laden. In sum, it's inconsistent, but largely worthwhile. It was really more of a heist/caper series than a spy series; it was inspired in part by the heist sequence in the movie Topkapi, but since broadcast standards of the era would never have allowed presenting criminals as heroes, they made it a spy show so that the crimes and frauds were being committed against enemies of the US in the name of peace and security.
For the most part, it was all about showing the mechanics of how the capers unfolded step by step; in a way, it was similar to CSI in that so much of it focused on wordless sequences of characters doing meticulous work, with prominent musical accompaniment. The music was a huge part of the show for most of its run, though the last two seasons rely mostly on stock music. And one thing that was not a huge part of the show was character development. The main characters were deliberately kept as ciphers, always subsumed beneath the roles they played each week, with at most one or two special episodes per season breaking with that pattern. The exceptions to that were the early first season, which did focus more on the characters before the decision to downplay them, and the fifth season, which was in many ways a deconstruction of the show's formula and delved much more into the characters, as well as doing more format-breaking episodes, e.g. capers that go wrong or crises that crop up after an offscreen caper ends.
For the most part, though, the series was relentlessly formulaic and had little suspense, since the plans usually went off like clockwork exactly as the team arranged them. Most of the suspense came from the fact that we didn't know every detail of the plan in advance -- they'd set up various tactics and gadgets they planned to use in the opening, but we wouldn't find out how they'd be used or why until later. I like to say M:I was more of a spy procedural than a spy thriller. Basically, it was completely unlike the movie series of the same name. The first movie started out recreating the standard formula, then completely blew it up and replaced it with a fast-paced conspiracy-thriller formula that pretty much all the sequels have followed. The only movie in the Ethan Hunt series that comes at all close to the spirit of the show is Ghost Protocol.
Although in the last two seasons, it abandoned the spy stuff (which was falling out of fashion) and became almost exclusively about battling organized crime in the US, something that they'd only done occasionally in earlier seasons. Which made it strange and illogical that they kept using the trope of secret message drops and self-destructing tapes to deliver instructions. That made sense when they were an off-book black-ops team doing extralegal dirty tricks that would require them to be disavowed if caught, but not when they were working openly and regularly with conventional law enforcement to pull stings on the mob. Although even the earlier seasons often glossed over the off-book/deniability angle and had the team cooperating with the police or other agencies.
So for the most part, it's not a particularly dramatically complex or challenging series, and it's slower-paced than modern TV. But it has a charismatic cast, often very clever capers, and terrific music.
