I've been using 'collage' (non-PRO) for quite a while now, and upgraded to PRO a few days ago.
Basically, what collage (both versions) have over most of the other collage layout apps is *Speed*. That's why I use it after a shoot or event - select 20 images, drop them on collage, and boom! Collage automatically creates a layout. No templates needed. I then use in-app zoom and pan for touch up - and I'm done. The layout is usually good enough to present directly, and it saves me a ton of time when antsy clients want to have a preview. PS can of course do the same, but that would take me hours for what I can do in literally seconds.
collage has one drawback, though - it's somewhat inflexible with its layout, and can sometimes be downright quirky when you edit it. And it does not allow overlapping images.
One major knock, though: no text. Although some templates look as if they use text, they in reality use pictures of text, cleverly added as a background image. I create captions in Acorn, and import them as pictures. Annoying.
Other than that, though, I really like the app as a *very* fast collage creator. If you have the time, try both versions because not everyone needs (or wants) the PRO version's additional features, and you can always upgrade later.