It supports single and multi-file downloads through HTTP, FTP (and Bittorrent, kind of).
To use it, you just click on a link that points to a Metalink file. Each time it will try to download a file from a different mirror. If it's a multi-file Metalink, a dialog will show up that lists all the files. Also using a different mirror for each file, each time, if possible.
If no file could be found on the first try and you click the Metalink again, all available mirrors will be listed (if there's more than one).
Advanced features like hash verification are not supported. Mostly since there's no way to access the download manager from extensions (yet?). So I'd have to write my own for those features, and I'd like to avoid that.