I've experimented with a reader magnet for the past two years. First I gave away
Beltane Fire. More recently I gave away
Samhain Witch. Both books were
only available for the newsletter exchange (except a paperback). They were short stories connected with my witch series. I offered the books for free on my website and in various newsletter swaps (many Bookfunnel) in exchange for joining my newsletter.
I can't quantify my results exactly, but I believe that by offering the books for free, results for new members were not much better than joining gift promos. Gift promos offer a book as a prize--they can be any of your books. These are run by various promo companies such as Bookfunnel, AXP, ILVN, Crave Books, etc (the most recent Crave Books promo garnered me 1200 emails for $5

). These promos usually offer one book as a gift, they don't generally offer free books to
everyone. You admittedly get a lot of junk emails and you have to regularly remove subscribers, but I think it builds your list much quicker.
I suppose that's the issue. I can get 1-2 readers from subscribers who loved one of my books (I also offer newsletter subscription at the end of all my books), but I'm not living long enough to see 10 new readers a year growing my writing career. I just don't think this strategy, or reader magnets, works well. I recognize that these are typically the best subscribers, but the number's still negligible. Yet many writers are doing it, so...

What do you guys think? I have 5000 readers in my newsletter. I average around 25% open rate. I write in paranormal fantasy/romance.