Ranked in these QuestionsQuestion Ranking
Pro It just works
GDM is dull, but it just works, and it is highly stable. It's easy to switch between environments, and it integrates really well with Fedora or other Gnome Distros.
Pro Always signs the GNOME keyring
Works with any desktop. Some display managers such as SDDM don't open the GNOME keyring at login.
Pro Not so difficult to customize
You just have to know which files to edit and you can do quite a lot.
Con Tied to Gnome
You basically have to use Gnome or one of its forks to use it properly.
Con Depends on GTK and its dependencies
Con Really hard to customize
You'll have to recompile Gnome's resource files in order just to change the login background.
Con Ugly UI
It is very simple and you cant change the layout.