Now with custom image sizes Wordpress is almost as powerful as timThumb / phpThumb. There is one problem however. The way Wordpress handles the resized images are chaotic and doesn't survive theme changes / image size changes.
The problem is that wordpress names the files like: image-600x400.jpg, instead of naming them image-medium.jpg or image-custom.jpg.
Is it possible to override this behaviour? It would finally make Wordpress manage image sizes perfectly! You would just need to regenerate your thumbnails with any regenerator plugin, but your URL's would all work! (Also the size information should be removed from the posts.)