It's a very agreeable movement and one I can see no problems with at all. I've always wondered why it's considered acceptable for men but not for women and I hope that this hashtag will have some effect. I've seen two criticisms to the movement and neither of them really hold any weight if you think about it, so I'll go over both of them now.
First, people have argued that it's rather hypocritical 'of feminists' to complain about things like The Sun sexualising women with Page 3 images of topless women only to then go ahead and sexualise themselves. Without realising it, people who have said this are actually proving the point that the #FreeTheNipple people are trying to make; there's nothing objectively sexual about a photograph of a woman with her breasts exposed and the point of the #FreeTheNipple movement is to change people's perceptions so that more people can understand that. Certainly, people can and do enjoy breasts in a sexual way, but it should be acknowledged that this isn't the only way of looking at them; it's because of attitudes like this that women get told off for breastfeeding in public.
Second, I've seen an argument that it's strange to try to desexualise breasts, because it's nice to enjoy them in a sexual way, like leaving delicious chocolates out of their wrappers (that's not my comparison). This also seems quite strange to me, because it suggests that people would become incapable of viewing breasts in a sexual light if they were to become desexualised generally. In fact, if somebody can't make the distinction between a sexual context and a non-sexual context, that's rather creepy.
And those are the only arguments against it that I've seen and, as much as I think about it, I can't think of any thing wrong with this movement at all. I hope it continues to grow in numbers and influence.