You're not the first person to feel that way about Jack, which is odd to me since he's easily one of my favorite characters.  He was always trying to help others from the get-go, and was always the one person you could trust.

While I disagree that it seems made up as they go along and overly complex, and I respect your (and most everyone else's) opinion, the following is why I absolutely love the later seasons of LOST:

*Spoilers for anyone who's never seen LOST and is interested in checking it out sometime... stop reading now!*

Part of what makes LOST so genius is the way the story is told.  They could have started the pilot episode from a chronological standpoint, long ago with the story of Jacob and his brother and how they came to be deity type figures that represent "Good" and "Evil".  "Evil" (Jacob's brother) is trapped on the island and wants to leave, and the only way he can do that is to kill "Good" (Jacob) and put out the light of the island, which represents the good in mankind, which Jacob protects.  Basically "Evil" is a pesimist who believes that since man is corruptible, man is not worth existing.  "Good" tries to prove him wrong over the years by bringing people to the island, but "Evil" keeps proving his point, and eventually the people always end up dead.  The thing is, "Evil" cannot kill "Good", and vice versa because of the "rules".

Being at a standoff, "Good" (Jacob) proposes they play a game with humanity:  If "Evil" (Jacob's brother) can beat him at the game and have him killed (thus proving man's lack of redemption), he gets to leave, which is the end of mankind.  And while "Evil" has free reign over the fate of the majortiy of the players, Jacob chooses a select few to be "candidates" to take his place if he perishes, and "Evil" cannot harm the candidates himself.  The entire show has a beautiful religious metaphor, but doesn't bombard you with it.  Conversely, the scientific aspect is very present, proposing interesting explanations for things, such as the "afterlife" being an alternate timeline/reality because of a hydrogen bomb explosion (there is actually scientific merit behind this, albiet theoretical).

If the show was layed out in this way from the beginning, I wouldn't have enjoyed it nearly as much.  Instead, it very wisely begins with a plane crash on a mysterious island and gets the viewer invested in the characters and their story of survival.  The greater story arc is not elaborated on at all until those later seasons, and going back and watching it again, you realize that the entire story of LOST is about how "Evil" devises an elaborate plan of finding an easily susceptible castaway (John Locke) and filling him with false hope that he is special and chosen.  "Evil" has the power to appear as the image of anyone who has died, and can also realize their thoughts and past, and uses this to build up the importance of Locke to everyone on the island as a savior of sorts.  Locke is tricked into thinking he must die for the island, enabling "Evil" to assume the appearance of Locke and coerce someone (Benjamin Linus) into finally killing Jacob, as he could not do so himself.

The cool thing is, Locke and Jack are constantly at opposite ends of philosophy.  Jack is a man of science, Locke a man of faith.  Jacob and his brother are much like god (who observes) and the devil (who tempts):  While "Evil" resorts to tricking Locke and using his faith against him, Jacob instead believes that Jack will eventually find his own faith and prevail for the good of mankind.  The most important thing in the show, however, are the characters, and I'm really glad they focused on this aspect for the beginning and ending, making for an incredibly creative and entertaining show.

Yes, yes he is...

http://www.metacritic.com/tv/lost/season-1/user-reviews

Despite a few mediocre and even a couple of negative opinions, both the meta and user averages remain pleasantly high... and far from "garbage". 

If you look carefully you'll see that the grades got lower with each season.

You'll also notice that it doesn't change the fact in my point in the least.

The fact that fans (and especialyl critics) liked the show less and less with each season is irrelevant?

Try reading my first post again...

The user averages never fall below 8.0 for any season despite the people who gave it negative reviews.

Which brings me to your last response above to @Bunyip:  Looks like most would say it is your opinion that is wrong.