I'm using the plugin next gen in a special multiblog installation.
The problem is that nextgen doesn't supoort absolute path for gallery directory nor respect the WP_CONTENT_DIR and WP_CONTENT_URL.
To make NextGen working with my special installation i change the WINABSPATH in the core nggallery.php.
This is a bit problematic with upgrades
may nextgen in future will have the gallery pâth build against WP_CONTENT_DIR and not ABSPATH ?