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.
collagePRO is essentially the same, except that in is more relaxed with the layout (images can overlap, rotate), but it retains most of the layout quirkyness. The PRO version also has backgrounds (images, textures), different frame shapes (for those who need it - personally I prefer boringly rectangular frames) and a really, really nice paper format manager. To me, that manager, coupled with the auto-layout is the real killer function in the PRO version. I can create multiple exports (calendar, greeting card, web page) of the same basic layout instantly. The HTML export option is nice, but due to its heavy use of javascript can be slow for collages with lots of 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.