I personally am not religious. However, I have a modification of Realmfighter's criterion: So long as the religion causes no/minimal external harm, let them have their harmless fun.
Of course, even such a simple criterion as this can cause lots of arguments. For example, I used to go to a fairly liberal Methodist church; that seemed like a fairly good religion by this definition. They were careful to be tolerant and such, and did quite a bit of volunteer/charity/donation work. Catholicism is slightly further down the scale; it has some unhelpful things that are followed with varying degrees of fervor, depending, but plenty of Catholics are perfectly good people, and Catholicism can get quite a bit of charity work done due to sheer size. Islam is an interesting one; it is currently fairly high on the "external harm" scale, but I could see it becoming something on the same level as the various other modern forms of Christianity. It would just take quite a lot of effort and time, most likely.
"Religion causes external harm" in and of itself can be difficult, because while much persecution is effectively because one group is different than another group in some way, religion is one of the more common rationalizations for this persecution.
At any rate, religion is probably not going to go away entirely for quite a while, if ever. The fact that basically every society ever has had some form of religion indicates that it is something humans are prone to; I am uncertain of the precise reasons why, and would be interested to hear any hypotheses.