I, for one, don't really consider myself to have a religion. I somewhat believe in something, but not someone, and I don't worship it or anything. Anyway, I do and don't agree with 'blastamy' =P.
I believe it can be alright in certain circumstances. I don't like how many religions push their beliefs on you, as well as force you to do things you don't want to. Now, this doesn't need to lead to insulting and mocking, but I can see how people will do it. For example in Christianity, you give something up for lent; someone who is not Christian might be like...how is this benefiting you, or the religion.
I don't agree with it when its just basically ignorance. There is no need to be like, "Jesus sucks, I'm better then him." If you don't believe in him, there is no need to put down others' beliefs.