We just wrapped up our first Girl Develop It course in Sydney tonight. When I was first planning the course, I had males ask if they could be students and TAs, and after some consideration, I said no to them.
Part of me wanted to prove that we could pull it off with an all female ensemble. We ended up enrolling 40 female students, bringing us to full room capacity (daisy-chained power cords, ftw!), and enlisting the help of 6 highly skilled female teaching assistants, from web standards wizards to JS experts. I thus concluded that lack of "womanpower" was clearly not an issue.
The other part of me wanted to see if we could indeed have a better learning environment by having it be all female, as we suggest is the case on the Girl Develop It website. I was the teacher in this course, so I can only give my perspective from the front of the room. But, I have to say, I liked it. I am a straight woman, so when I am giving talks to the mostly all-male crowds at most tech events, I sense a small part of me is trying to impress a small part of them ("that way"). It's not something I'm very conscious of, as I'm usually fairly empassioned by the ideas my talk, but it is there nonetheless. When I am speaking to a group of all females, I am motivated only by the desire to educate them and not by any hidden desires. I played the part of the teacher in this course, but at the same time, I am also a student in an Afro-Brazilian dance class which is largely female. Similar to my reasoning for enjoying the absence of boys in the web dev course, I find that I enjoy the dance classes more when it is just us girls. I can shake my hips without worrying subconciously about impressing the boys in the class and having my performance affected by subsequent nervousness.
On a related note, it's nice to be in an environment where we can talk girl stuff and bring up "risque" topics without worrying about making boys feel awkward or wondering if they'll misinterpret our language. In dance class, we often make up rhymes about our "boobs", "hips", and "asses", and they help us learn the move... but it always feels a bit odd to teach them to boys too. That sort of thing doesn't happen as often in the web development course situation (well, maybe during the after-drinks :), but it's nice to have that kind of environment just-in-case.
Finally, it's cool to meet local women. I have to admit that I'm not that great at making friends with girls (I grew up more around males), so I typically only make them when I'm forced to. Being in an entirely female room definitely helps as a forcing function. :) I met a bunch of awesome girls during this course and the dance class who I probably wouldnt've met otherwise, and I'm looking forward to seeing more of them.
I know there are people who may argue that it is sexist to not allow boys into the classes, but I think that if you are going to go the "no boys" route, you should go all the way or you risk losing some of the benefits completely. This doesn't mean that I think every thing should be all girls - I will be actively encouraging the GDI students to come to mixed meetups, workshops, and user groups. It just means that I do see benefits to single-gender groups in some situations, atleast from my own personal perspective.