|So, I use it, but in a very limited way, actually. I use Aperture and the Nik Plugin Suite for most of my editing, but I got Acorn in a bundle some time ago--I use it for adding text to the image: it is very user friendly for that, and I can place the text anywhere (Aperture's watermark placement is pretty limited). Off the top of my head I would say that Acorn is very much like Photoshop elements, and it is frequently upgraded. I have had several free upgrades along the way. They just upgraded to v3, which they want some payment for. Since I don't use it for regular editing, I don't really need to upgrade.
In my limited use, I have found it easy to use and stable. Hopefully someone else here has made more use of it and can comment.