I just done my first project on C5. And this is part of my feedback, things I don't like about thumbnails system:

1. You can't set max height with auto width. Why is that? It's not hard to code, I've wrote function like this about 5 years ago.
2. There is no "crop" checkbox, it crops no matter what. But what if I need uncropped images of max height and max width for my template?
3. Thumbnails sometimes doesn't create at all. What? Whhhyy?
4. You can't change Required flag and it even doesn't do anything. It creates a ton of unnecessary thumbnails for every uploaded picture. What if I need 100x120 thumbnail only for one custom type of block? We should be abble to disable auto-creation of some thumbnails. And then you guys need to add a function that will allow to check in block controller if image thumbnail was created (by thumbnail handle) and if not, create it and return a url. That's a right approach.
5. When editin on a page, and you want to insert an image, you can't pick from one of the thumbnails, you can insert only original image.

So, thumbnails system disappointed me the most... as you can tell. On my next C5 project I'll probably have to add my own function that create thumbnails, that can crop or not crop, that have max-height attribute with auto width and that actually creates thumbnail of right size no matter what. The only down side - user will not be abble to change thumbnail if it croped bad, this sucks. So I'm hopping that developers will fix their thumbnails system...