Well...finally saw the movie. And...well I damn near cried when Stu started confessing to his wife and Pam and the others. It truly touched my heart.
But enough with my emotions. I really liked the flick, to tell you the truth, so much that I almost forgot why I decided to see it in the first place. But I paid attention, and this is what I got out of it:
Stu was used to represent the whole of mankind, as he commits each of the Seven Deadly Sins, all in one form or another, as follows:
PRIDE, or vanity, is excessive belief in one's own abilities, that interferes with the individual's recognition of the grace of God. Stu imbues himself with a "false importance", donning expensive Italian suits and the like. But his importance only runs as deep as his clothes in reality. This is shown when he tries to bargain his way out of death with the sniper.
ENVY is the desire for others' traits, status, abilities, or situation. Stu's pride is perhaps an effect of his envy of those above him, which is seen in his desire to dress like them, etc.
GLUTTONY is an inordinate desire to consume more than that which one requires. Stu loved attention. He fed off of it like candy. I took this as a form of gluttony.
LUST is kind of self-explanatory, and quite obvious in the movie as well: Stu's affair with Pam.
ANGER is also self-explanatory, and also very obvious. Stu expressed it towards the pizza guy, the sniper, and others.
GREED is the desire for material wealth or gain, ignoring the realm of the spiritual. Stu was so wrapped up in his job, trying to make his cut, that he made no time to reflect on what he was actually doing.
SLOTH is the avoidance of physical or spiritual work.
Stu left it to Adam, his assistant, to do all the hard work, while he hands out lies and misconceptions. On the spiritual level, he apparently makes no effort until his encounter with the sniper.
Whew...ok. And that was just Stu. I can almost safely say that everyone has committed at least one of these sins on one occasion or another.
As for the others, they represented some aspect that either help, hinder, or otherwise affect mankind and it's decisions.
Phone = Last connection to God
The Sniper = God
Pizza Guy = Denial of God
Pizza = Life
Destroyed Robot = Warning from God????
Hookers = Distractions
Leon = Satan
Wife = Love
Pam = Lust
The plot begins to thicken when the pizza guy arrives. He offers Stu the pizza (the bread of life, or eternal life) which Stu refuses. This is when the sniper(God) calls and gives Stu a warning (destroys the robot) which with Stu begins to take seriously. Like God, the sniper sees all, knows all, and chooses to take action in secrecy. After the warning, Stu is distracted by the hookers, and they cloud his judgement. Then Leon (Satan) appears and tries to get Stu to hang up the phone. In doing so, he would destroy his last chance, and end up dying, on a literal note but it is his salvation that dies on a metaphorical note. The sniper strikes down Satan in Stu's weakness, giving him one more chance.
I'd go into more, but I'm probably over the post limit already. Besides, I pretty much agree with what biteme2514 has said about the rest of it.