This is going to be a quick review:
The 'Melancholy of Haruhi' cycle, which is only 6 episodes of the series is somewhat like crack made from little orphans in Mexico. After it's ripped away from you, your hopes and expectations that everything will be awesome from now on are shattered, and it was so addictive you couldn't care less how many little orphans it would take to keep it coming.
Unfortunately, the main driving point of the plot: the relationship of Kyon and Haruhi comes to a screeching halt after these episodes, and what we are left with is a paper thin plot that comes dangerously close to making good performances and characters, filler material. Namely, the plot soley becomes keeping Haruhi happy. Now I know A LOT of people would be pissed if I called these "gems" filler, and they're not. But I would say, because of the potency of the previous episodes- you know, with things like an unlikely romance developing, people saving each other's lives in epic battles, characters revealing secret identities, impossible childhood fantasies becoming validated, AND a comedic tone- these afterthought episodes just aren't up to snuff. And you can also forget any possiblity of this being a real romance. It was just a tease.
What I really don't understand is that this could have been (and seemed to be headed in the direction of) an anime I could really say was one of my favorites. That is, to selfishly say, Kyon and Haruhi's relationship could have continued to develop, with Kyon becoming more outgoing and positive and Haruhi becoming more mellow and empathetic towards (at least on the inside) Kyon. Perhaps Itsuki and could have revealed more of their intentions instead of giving us more end-of-the-world-sci-fi monologues backed by gregorian chants(Oh, yes, I do understand that some of this was funny at first. But should it have been employed this much? I mean how many explanations does Kyon need to hear about Haruhi being God before he finally understands (and then apparently forgets? later)). And perhaps, Asahina could actually have a history(future) we actually get to know about. Apparently, I'm asking for too much here.
If I had to read between the lines I would wager that these flaws were all about risks that could have been taken by the animation staff but weren't. Like, "Let's not mess up our perfect formula of character interaction. It works so well," because after the character-development episodes it feels like they pull back a little and start looking inside those episodes for ideas to flesh out later. The same topics keep getting rehashed. Meanwhile, the plot is in a stranglehold; careful not to talk about anything except keeping Haruhi from getting upset. The plot does go somewhere, but not very far and in a somewhat circular fashion. Honestly, some would say these are filler episodes, but it would be more accurate to call it stagnation.
Despite all of this, I actually enjoyed the relationships and chemistry between all of the characters. The skeptical and passive aggressive Kyon with the fascist and fanatical Haruhi make for VERY entertaining bits, while Asahina and Itsuki are parodies of the submissive-ecchi-crybaby and the charming-yet-insidious-pretty-boy, respectively. They tickled my anime-critic fancy in a pleasant way. Let's not forget the emotionless-quiet-girl who is embodied by Nagato. Keep in mind this series never devolves into a pure satire or mere collection of parodies, and reminds us accordingly with frequent meaningful (at first) character interactions and dialogue. In short, I felt the beginning of the series affected me so strongly, that I couldn't say you SHOULDN'T watch this series. But don't say I didn't warn you.
Note: If you also think this series get's stuck in a rut/sidetracked and are looking for more character development in the sequel series, you won't find any. Oh, and good luck with the "Endless Eight". I feel sorry for you already.