Ability to work on multiple platforms, important for me when reading on the go.

I was a long time user of the old Papers (when it was under Mekentosj). I was a bit nervous about the upgrade and having to pay a subscription, I have been bitten before when a company has purchased an indie dev software I have used. However it was wonderful - works great and better than before and I like the web based access. It does everythign well and I use it every day. iPad apps also work nicely and sync to the cloud. A very good product and well worth the cost. For the record I am a senior Professor at a University with a library of hundreds of papers. Customer support is also excellent, I've commented on a few issues, some of them have been fixed but all are very positively and quickly replied to.

Makes it easy to annotate papers and keep these organized. I like that usually figures and supplements are included as well.