Ranked in these QuestionsQuestion Ranking
Pro Lightweight and fast
It's a lot more lightweight than the most common GTK or QT based login managers.
Pro Matured Software
XDM is not new, it has been in development since the 80s and is a very stable
Pro Allows several seats
Very customizable and portable, although with not as advanced effects.
Con By deault you can't choose desktop environments
by default you can't choose your sessions/desktops, however there are plenty of tutorials on the web to add a selection.