Perma-free is the single most powerful current marketing and discovery tool for indie authors. I have two full-length novels, JET and King of Swords, perma- free. Both series sell a lot of books, as in around 70% of my total sales, maybe more. I recently took my Assassin prequel off perma-free and am trying King free for a month to see if the retention rate is higher - King is a full-length, proper novel with a more robust feel to it than the prequel, which I'm now pricing at $2.99 as it's a shorter work.
Additionally, my sales in UK have gone from 50 a day to 150 a day since making my two novels free. It's all about discovery, and barring an unlimited ad budget and Dan Brown saying your book is slathered in awesome sauce, perma-free is as good as it gets, IMO. But it's only worth it if you have at least two more in the series and can get another out in fairly short order. If only two books, I question the merit of it, and I'd just as soon bite the bullet and hold it on perma-free until I had book 3 released. That's my take.