I like this summary of the problems with Pascal's Wager from Godless Geeks.
From "Why Atheism?" by Mark Thomas
There is an argument for belief in God that is called Pascal’s Wager, named for Blaise Pascal who conceived it. The argument goes like this: Either there is a god or there isn’t. If you believe in God, and God exists, then you win big time and go to Heaven. If you don’t believe in God, and God exists, you lose big time and go to Hell. If there is no god, then you haven’t lost much by believing. So the obvious choice is to believe in God, because it’s simply the best bet.
Pascal’s Wager has several faults. The biggest problem is that it’s not a proof of any god’s existence; it’s just an argument for believing, a method of extorting the gullible thru fear. Like many other such arguments we have discussed, it also fails to denote exactly which god it refers to. Pascal’s Wager could be applied to any god that offers rewards and punishments. Taken to the extreme, following the wager would necessitate betting on the god with the worst hell, so it could be avoided. It's impossible to know which god to worship, and which (perhaps jealous) gods to spurn. I doubt if many Christians would convert to Islam if the wager were presented by a Muslim who told them that Muslim Hell is worse than Christian Hell and Muslim Heaven is better than Christian Heaven.
Pascal’s Wager assumes that the chosen god's mind is knowable, and that he doesn't mind people believing in him for explicitly selfish reasons. Perhaps he actually prefers independent thinkers such as atheists, not obsequious followers. Since the Christian god Yahweh is on record as having lied, there's no way to know his intentions. It would be quite possible for a true believer to discover on Judgment Day that the destination was not Heaven. Yahweh, in his infinitely mysterious ways, had other plans; and there would be no appeal or debate with an omnipotent being.
Another problem with Pascal’s Wager is that it implicitly assumes that the odds of the two possibilities are similar. Since the odds of the Christian, Jewish, or Muslim god existing are zero, the wager creates a false dilemma. The wager even goes against the doctrine that many religions have where gambling is sinful. Note also that the existence of the wager and the fact that so many people think that it's relevant illuminate the lack of actual evidence for God.
Pascal’s Wager also depends on the idea that you don’t lose much by believing. This has been false for many who have trusted in their god for help or guidance, instead of seeking reality-based solutions. People have fought, killed and died for their belief in their god. Far too many have died because they (or their parents) chose prayer instead of medicine. Swords, bullets, poison, and poisonous snakes have killed many who thought that they were protected by their god. Even without these more dramatic effects, believers often devote significant time, energy and money to worshipping their god.
Beliefs in a god (and the often concomitant ideas of divine punishment and reward) too often make people more willing to accept inequalities in this life. Low-paid factory workers and slaves were taught that their rewards were in the afterlife, so they should be meek and obedient in this life to ensure their (imaginary) rewards. Even the factory and slave owners could think that they were part of their god's divine plan, and thus deserved their earthly rewards.
God-belief has real expenses that can be large or destructive.
The last problem with Pascal’s Wager is that it completely ignores and even denigrates intellectual integrity and honesty; the wager assumes that people can believe something just because they want to. As an example, let’s talk about belief in Santa Claus. Don’t we have more respect for a child who figures out that Santa doesn’t exist, and says so, rather than continuing to lie so he can get more presents? It’s a sign of growing integrity and maturity for children to stop believing in Santa. Similarly, adults can give up belief in a god when they realize that there’s no real evidence for their god. Christians can quit being “sheep” or “children of god” and become intellectually honest.
The loss of intellectual integrity and honesty engendered by Pascal’s Wager gives some insight into how apparently rational people can behave so irrationally. By accepting the wager, they have (perhaps implicitly) given up these important traits."