Everyone will tell you to go play face it and blah blah blah, but I'll tell you why matchmaking is actually amazingly fun.
1) you don't have to be good (like seriously, you don't have to be very good at all, just know how to operate a mouse and keyboard and if your playing a class know the basic skills like how to do a basic rocket jump or when to heal who)
When you play face it or other lobby sights, there is an unspoken expectation for you to at least have decent dm and game sense. In matchmaking, there is no such thing. People might still yell at you for runing spy to mid, or not healing your team, but ultimately there is virtually no pressure in matchmaking as opposed to lobby websites.
2) no class or weapon bans means you don't have to be ridiculously good to keep up.
The current 6s meta has been refined and fine tuned over years of experimenting to find the best blend of fast paced action and versatility. This formed the highly mobile and high damage dealing lineup of 2 scouts 2 soldier a demoman and a medic.
This is definitely NOT the case in matchmaking. People who think this is the best class combo don't know what they are talking about. Regular 6s has class and weapon bans which makes high mobility classes better then others. In matchmaking, weapons like the GRU and the whip make heavy white powerful.
heavy is a very low skill class which is great for less experienced players to play without hurting there team.
Matchmaking meta would probably be something like this: 2 medics, 1 demo, 1 heavy, 1 scout, 1 soldier.
Good blend in both mobility and defensive structure, with the extra medic and the heavy.
So basically, just play heavy if your bad.
3) it is very fun to play with your friends. Matchmaking isn't as serious as lobbies, so you can have tons of fun with your friends pulling of wacky team comps and strategies without any backlash.
You can add me if you would like to play with stacks. Matchmaking is going to be drastically improved over the next few months which will hopefully draw more players in, and ultimately increase the popularity of 3rd party competetive tf2.
steamcommunity/id/ReznorALT