The app is pretty straightforward. You select a group of photos and arrange them in a basic preferred layout, add a caption, upload to Photoset, and then you can share the link to the uploaded image or share it on Twitter. To use the app, you do not need a Tumblr account, Photoset keeps the link of your uploaded montages private until you share them with friends.
The simplicity of the interface looks to have appeal, and there is no mention about integrating the ability to share the images on Facebook. Based on the spec sheet, it has room to develop versus some of the other apps available. However, it is version 1.0.4, so updates to the application are certain to be forthcoming. The app is available now on iTunes.