"is it legitimate to criticize religious beliefs in others?"
I suppose any criticism is legitimate, but what is the intent of the criticism? Hopefully it is to bring to surface an inconsistency with what you think is false, or inaccurate, or simply superstitious.
As with most areas, self-criticism is a key to a well-developed confidence, and religious people who criticize their own beliefs are usually better at explaining to others exactly why they believe what they believe.
Many times a skeptic will criticize a religious person because that skeptic has a preconceived notion as to why the believer is in line with a certain way of thinking. And in some cases, belief does start out as superstition, only to be illuminated later in life as to that belief or superstition's usability in life.
Take prayer for example. The skeptic says, 'Why pray? It doesn't work. You won't get what you want. God doesn't even exist.' To which a religious person might answer, 'I do not pray to ask for things, but to express to a higher power my desire to be a better person, or to express my worries and desires to help my friends or to think of a way understand an issue that has been on my mind. I have found an applicable use for it to which I use because in the long run I have found it to be helpful in times of distress and confusion.'
I will have to find more of the thought-provoking discussions I partake in and post them here. Or, if you're a registered user of Reddit, feel free to add me as a friend.