VLC player is great because it is extremely versatile. While it lacks the polish you might find in another media player (and thankfully, also the bloat) you can get it to play *nearly* any type of multimedia including DVDs (VOB) and Flash Video (FLV), and it can even transcode between a huge variety of formats. It is also good at playing incomplete or damaged files, including partial downloads and damaged DVDs which can be handy.
It runs on a nice variety of platforms as well.
It's not my primary player, but it's a reliable workhorse and a must to keep around.
It simply works, plays everything I throw at it, can make screenshots, step by frames, play streams and more. I'm giving 4/5 because it's a bit slower to start than some other players and doesn't blow my mind with anything extra awesome, but it really is a very good player.