There's debate in feminist circles (because feminists are not the Borg, they all have differing views) as to whether men should call themselves feminists, or whether they should go with pro-feminist/feminist ally. There are surely many reasons for and ways of framing this, but the big one is co-option of the movement.
Feminism was created by women, for women. (A non-oppressive, equal society would be awesome for men too, but the primary mover was the inequality, wherein men are the primary power-holders). Right now, men still have the majority of the power. When a man speaks, people listen much more willingly than when a woman speaks. If I come in and say "I'm a feminist, and by the way, here's what I think feminism is about and how it should be oriented', I run the risk of shifting it away from the primary concerns and desires of the people it's by and for.
That in mind, though, we also should be using the people-listen-to-us privilege to ensure people listen to feminists, and ensure anti-feminist men don't get to act like they're speaking for all men. No, Mr. Women-Are-Oppressive-Men, you are not standing up for me, you do not speak for me, and I do not support you simply because I'm a man and you're a man and "those mean women are oppressing us".
(This is very different from interrupting "Men keep harassing me and we need to address it" discussions with "Hey, wait, not all men are harassing you! I'm not harassing you! See how great I am? Let's talk about how I'm not harassing you instead of how to stop those who are.")
So do I say I'm a feminist? To the general public, yes, absolutely. I want people to see that it's a movement they should identify with and support, even if they're not the man-hating straw woman. But when I'm in the midst of people with a more sophisticated understanding, who actually know what feminism is* and know the terminology, I'll go with feminist/ally.
* If you believe in, support, look fondly on, hope for, and/or work towards equality of the sexes, you are a feminist. (Or an ally. :-p )
Original version of this write-up is below because it's not too bad:
Now hold on. If you consider a feminist to be someone who thinks men and women to be equal and to deserve equal rights, then I am a feminist. If somebody asks me if I am one should I lie about it? Why?
So many women are afraid to proclaim themselves feminists because they're afraid people will assume them to be feminazis. the more people who stand up and say "I'm a feminist" the less people will be afraid of unfair comparisons.
I'm a feminist