Yeah, it has MTX. You don't have to pay a single penny, if you chose to go that route. Everything you can get with money, you can get with just playtime. You earn IP (currency) throughout your matches, which you can spend on purchasing a character, or purchasing runes (pretty much must-have enhancements, based on tier your account is in).
You can also purchase skins, champions and ip/xp boosts for real cash. You will rarely go up against people outside of your tier, so it's really not necessary to play catch-up.
Each week, they have a line-up of free characters that you can play, randomly picked out of the total pool. Obviously if you wish to keep a character that you liked that week, it needs to be purchased to be on your permanent roster. If you don't wish to fork up the in-game or MTX currency, you can continue playing a different character from the next week's rotation.
It's a very neat system, and I'm not a gambling/addictive type. I just really like the game, therefore I chose to support the developer. I've got a fair share of skins, as well as all characters, but one, unlocked.
It's a really fun game to play with friends. It seems casual, but everyone gets really into it.