(Lots of spoilers here, so proceed with caution)
I have to admit that my initial motivation for finally sitting down to watch this entire 5-season, 105-episode series was the chance of seeing a hot woman like Jennifer Garner kicking all sorts of a$$ and performing wild stunts while being dressed in colorful wigs and skimpy outfits that reveal her fit body. And while that is, undeniably, part of the show's appeal, "Alias" is much more than that. In fact, my favorite character in the series turned out to be not Sydney Bristow, but the evil (?) mastermind Arvin Sloane: Ron Rifkin's exceptional performance turns this character's journey through the seasons into an almost Shakespearean tragedy about ambition, sin, loss, guilt, redemption, obsession and fate.
Season 1 is the most fun and traditional of "Alias": the emphasis here is on the action, the gadgets, the globe-trotting, and Sydney's efforts to avoid being exposed as a double agent for the CIA. Still, the show does not shy away for the dark sides of espionage (after all, the heroine does lose her fiancé AND some of her teeth in the first episode!), and Jack Bristow's (Sydney's father, also a double agent for the CIA) often morally questionable tactics remind us that this is a world where the lines between good and bad are not always clearly defined. My favorite episode: "Page 47".
Season 2 introduces Sydney's long-considered-dead mother, Irina Derevko (fascinating performance by Lena Olin), which creates further emotional and moral complications for Sydney and Jack, and halfway through it changes the initial premise of the show, taking off in a different direction. Family dynamics, unexpected enemies and intense action make this season arguably the most popular of the show. My favorite episode: "Passage".
Season 3 is my personal favorite, because it is the most Rambaldi-driven, puzzle-like and plot-heavy, because Sloane is at his most ambiguous, and because a twisted, amoral couple (Sark and Lauren) steals the show from the "official" leads, Sydney and Vaughn. Of course, these are the reasons that some people consider this their least favorite season; decide for yourself. The action begins to rely more on guns and less on kickboxing from this point on. My favorite episode: "Conscious" (special guest star: David Cronenberg!).
Season 4 has a frustrating start, because it puts most of season 3's plot lines on hold, and goes off in a series of "stand-alone" episodes that don't even end on the series' trademark cliffhangers. With that said, some of those episodes are enjoyable, the introduction (though technically it was done in season 3) of Sydney's little sister Nadia (the incredibly beautiful Mia Maestro) works well, and the pace does pick up in the second half. Trivia: Jennifer Garner made her directorial debut in this season with "In Dreams", and it's easily one of its best episodes. But my favorite is "The Index".
Season 5 is (or should be) the most controversial, especially for the way it handled the apparent death of a main character. It is also shorter than the others (17 instead of 22 episodes), which makes parts of it, especially near the end, feel rushed. The ultimate Milo Rambaldi secret is finally revealed, but most loyal fans will have already guessed it. Garner was pregnant in real life - and in the show - during the first half of this season, so most of the action was handled by the other characters, including some new arrivals who all have their merits, but not quite the personal connection to Sydney (or Sloane!) that Nadia had. The advantage that season 5 has over season 4 is that it returns to the puzzle-like, one-clue-leads-to-the-next format of seasons 2 & 3, which means very few slow spots. My favorite episode: "The Horizon".
"Alias" has its drawbacks: the main one is that it often requires MASSIVE suspension of disbelief, since nearly every character (not just Sydney) has abilities (physical, intellectual, technical, or all three) that are close to the supernatural. At the same time, "Alias" never - or almost never - forgets the motto: characters come first, action comes second. The action scenes - especially the vehicle chases & crashes - are often movie-level, but it's the dialogue scenes, and the superb acting from everyone in the (regular & guest) cast, that draws you into this world.
At its best, "Alias" is an extremely addictive TV series. At its worst, it's still better than, say, most of the James Bond movies!