This is an argument I've been answering constantly ever since I started here. No matter how many times it comes up, there are always those who think it's a brand new, ingenious zinger which will take us by surprise. I'll address it once more and then refer back to here in future.
There are four main issues with the Wager, any one of which would render it nonsensical or inadequate.
1. It presents a false dilemma: that either God exists or no god does.
There is an obvious third option, namely that any deity besides the expected god exists. If the real deity is Thor, for example, the punishment for Christians is infinite (possibly worse than for atheists, who at least do not worship a rival god).
Humans have imagined something like 20,000 different major deities or equivalents so far. Together with the countless ones we haven't thought of yet, there are an infinite number of possible gods. Without evidence for any particular god, all gods share equal probability of practically zero, and the probability of a particular god existing is infinitesimal compared to the probability of one or more rival gods, so worshipping any god is a hugely bad bet.
The response to this, I know, is to argue that there is evidence for your particular god and not for any of the others. That's a valid response, if true. However, if you have proof positive that your god is the one and only there's no need to mess around with probabilities, so you don't need to use Pascal's Wager in the first place. Just push your evidence instead.
2. If there are no gods, you don't lose nothing by living as if there is one. You lose plenty.
You spend hundreds or thousands of hours attending religious services. You give money to organisations whose primary purpose is not to help people but to convert them. You prevent yourself from doing some things you enjoy, not because they hurt anyone but because a book told you to. And so on.
3. Belief in gods is not a choice.
A person either believes there's a god or doesn't. This may change, but it's not a conscious decision by the person. Her or she has to be convinced, or else no longer convinced, one way or the other. The idea that it's beneficial to believe in a god does not support the idea that there is one. They're two independent issues.
4. Any decent god would spot a faker.
This is related to the third point. If an atheist were convinced that it's beneficial to believe in and worship God, he or she could certainly worship, but would still not believe. The worship would therefore be insincere on a fundamental level. It'd be a farce, maintained to give the appearance of belief. Would the Christian god, for example, accept this lip service?
It's said by some religious folks that if you pray with doubt, but pray with sincerity, belief will come. I don't doubt it; if you pray as if there's a god there for long enough, you may manage to forget that there isn't. If brainwashing yourself like this is the only way to believe, however, are you really doing the right thing?
In short, Pascal's Wager uses an incomplete and incorrect premise, and is useless to nonbelievers even if they agree with it. Blaise Pascal was a brilliant mathematician to whom we are all indebted, but his apologetic really let him down. Let this one go, people.