This forum is open to all users, so I hope that some PeerJ authors will choose to provide their reasons as well. However, from our perspective there are many reasons why an author would choose PeerJ over other Open Access journals (although of course it depends on the specific journal).
#1 - PeerJ is significantly cheaper than most 'pay to publish' open access journals. Publishing plans start at just $99 which allows an author to publish 1 article / year for free, for life (provided their co-authors have also bought a paid plan at the right level). Compare this to typical APC fees of $1000-$5000 at many open access journals.
#2 - Authors who have been through our process have had a universally positive experience. See the results of our author survey in which 92% reported an overall positive experience (42% of them reported that their experience was one of the "best they had ever had"!).
#3 - We are fast. We are currently getting first decisions back to authors in just 24 days and all decisions are made with the input of the expert Academic Editor and at least 2 external peer-reviewers. Compare this to time-frames of months or more at most journals.
#4 - We operate optional 'Open Peer Review' (a system unique to us). This means that reviewers are encouraged to provide their names when reviewing (currently 38% are doing so), and authors are able to make their entire peer review history public on their published articles (currently 75% are doing so). This encourages transparency and allows authors to demonstrate the high quality process they have gone through.
#5 - We provide an attractive and fully integrated environment to submit and publish into. With a single logon, authors can submit a preprint to PeerJ PrePrints; gain valuable feedback; revise and update that preprint; then transfer it into PeerJ for formal peer-review; publish the final article with their full peer-review history; and gain yet more feedback on that article. And all of this is in a beautiful, user friendly interface for both authors and readers.
#6 - As an individual, every interaction you have in our system can be credited back to your profile. For example, your profile page shows which articles you commented upon, authored, reviewed or edited. At last, instead of your contributions to the process of scholarly communication going largely unnoticed, you are now able to get full credit for all of your interactions.
There are many other good reasons, but we will leave it to others to point them out perhaps!