Not too long ago, All For One mode was released for everyone on the League servers. Unlike the previous one, this one is based on the concept that both sides have the same characters. So, say if you choose LeBlanc, both teams are all composed of LeBlanc. Secondly, the normal version of this mode is now on Howling Abyss, rather than Summoners Rift, making the game much less serious and more faster paced.
However, I find this to be a con at times. Some characters that don't have much push power such as Braum or Shaco, which leads to long, sometimes boring games. The concept itself is quite warming, considering that it used to be a mod long before, and Riot is taking it as an idea, but it seems that the game gets fairly repetitive.
In a positive light, the game can be very fun when everyone is cooperating and doing entirely unorthodox builds. For example, I went tanky Zed and Yasuo and basically did better than some of the people on my team. It also allows people to experiment with different types of characters, considering that as long as one person has the character, anyone can vote for it in the champion select.
Overall, however, I find the game mode the funnest when you're with your friends, rather than solo. Not only can you laugh at the stupidity of so many ulti's and spells being used at once, but you gain a sense of pride when you use your abilities better than the enemy and win. I guess that this game mode is a 50-50 for me, some points are an absolute blast at times and can make your day, but it can also leave you frustrated when things just get bad.
So, I suggest you try it out. It's definitely fun the first game.
~Kuroneko