I've mostly taken part in them. It gives me a chance to check out things I wouldn't have read otherwise. If there is something in there I would have purchased anyway, I either buy something else by that author (if they have anything else available), buy the bundle myself, or at least buy something from Baen. If it isn't something I would have purchased anyway, then I feel Baen is getting what they wanted (which is for me, a person who purchases lots of books, to read new stuff and hopefully purchase more from a particular author or series they publish in the future). Most of what I read I probably got a book free (either via a giveaway or at the library) or cheap sometime in the past, liked what I read, and wanted to read more.
I have always expected them to put some kind of limit on it (such as only the actual purchaser getting to forward it) but, up until now, they haven't chosen to do that.