Even though this has been thoroughly debunked, it still gets used from time to time. I saw it used earlier this week so I figured it would be a good time to make my own post about it. If you want to see some background the wikipedia article and iron chariots article are both quite good.
According to the argument, there are 2 possibilities, either God exists or he doesn't. And you have 2 options, you can either believe in him or you can not. Combine these together and you have 4 possibilities, either God exists and you believe, God exists and you don't believe, God doesn't exist and you believe, God doesn't exist and you don't believe.
Now let's examine the possible outcomes, suppose God exists. If you believe, you will get infinite reward, if you don't believe you get infinite punishment. Obviously in this situation believing is the best option. But what if God doesn't exist? Whether you believe or not doesn't matter, in both situations you die and just cease to exist so they are completely equal. The conclusion is then obvious, Believing is either neutral or positive (and when it is positive it is infinitely positive) so the only logical conclusion is you should believe. (note: there is an alternate version on the wikipedia page that claims even if God doesn't exist belief is better than non-belief. That is not what I typically see in practice so I will stick to this version for now)
This might seem convincing the first time you hear it, it can certainly strike an emotional cord anytime infinite punishment is thrown at you, but it has major problems. Let's start with the biggest problem, it assumes that there are only 2 possibilities, the Christian God or no God at all. But there are many more gods that are believed in by people today and who knows how many others have fallen by the wayside to history. Many (most?) of those Gods also have consequences for not believing in them, if you believe in the Christian God and the Muslim God is real you will still wind up in hell. There is also the possibility that there is something like a God that nobody believes in and no one has heard of who has other rules besides what we know. What if the real god values critical thinking and would punish those who believe for a bad reason. What if believing because of Pascal's wager is the way to piss that god off the most? (Obviously I don't think this is true, but it is as much a possibility as any other God)
But there is another problem with this argument that is a bit more insidious, that is the assumption that there is no negative side to Christianity. There is nothing lost by being Christian, so even if you are wrong you should do it. If instead you want to argue that there are more positives than negatives in Christianity we can have that discussion, but the assertion that there is no downside to being Christian is garbage. So, let's talk about some downsides of Christianity:
1. It is expensive. I gotta say, it is pretty ballsy to claim that there is nothing lost by being a Christian and then to say you are supposed to give 10% of your check to the church. Sure, there is no penalty for it, but there is a lot of social pressure and guilt if you don't tithe. In the church I grew up in there was the occasional sermon that was completely devoted to tithing.
2. It is time consuming. We only have so much time on this planet, to have to go to church every sunday uses that time up. Again, it is not mandatory but it is expected and you will be guilted for not going. If you truly enjoy going then that is great, but many people go out of obligation, those people are wasting precious time.
3. It slows progress. Evolution and stem cell research. I've never heard an argument against these things that wasn't religiously motivated. Imagine the diseases we could have cured by now if our researchers could just do what they want to do.
4. Fanaticism. Religion can help people justify doing some crazy things. There are religiously motivated wars, suicide bombers, feuds, cults, faith healing, refusal of psychiatric treatment. The list goes on and on, this is just off the top of my head.