As a big Thor fan, I think this a very good idea actually. Here is why:

1) It's not permanent, so people shouldn't be worrying. If anyone reads the current Thor comic, they know this isn't permanent.

2) This will get readers who only read triple A titles to read something with a female leads. It expands people minds and expands the audience that reads female-led books which is very necessary.

3) Female books will be more likely to be read when they are released. People constantly complain that there aren't enough female books, when they aren't read. Marvel did try when they put Sif as the lead in Journey into Mystery. I thought it was a great book, but people refuse to read it. It makes no sense, but now we are tackling a much bigger audience.

4) I think this is great for the whole Thor mythos. Its a different take on a great title. It is an excellent opportunity to explore what it means to be worthy with Mjolnir.

5) The character looks sick, and this looks like a lot of fun.