I play it very often, and have been playing it regularly for the last 4-5 years.
I find unorganized games the most fun to play, in gamemodes like Jailbreak or Last Rabbit Standing.
Custom gamemodes like Assault or Assassination are fun too, although not hosted very often.
JJ2 is in my oppinion the best 2D sidescrolling shooter, with various gamemodes, levels/maps (you can even make your own!) and fun.
Here are two in-game recordings that I've made:http://www.youtube.com/watch?v=iXHsWLjYcKkhttp://www.youtube.com/watch?v=kGyM_kX7xkY
I've even ported the level editor, so you can use it in a browser (only Chrome).