For the sake of boredom here's a list vs the rest of megas. It seems like Charizard wins most, lol
Mega Beedril - Beedril wins if user is adamant. Must use protect first though, to allow speed to kick in.
Mega Pidgeot - Charizard wins unless user gets lucky with a confusion and hit
Mega Slowbro - Charizard wins. Very small solar beam chance not to OHKO
Mega Steelix - Charizard wins. No explanation needed.
Mega Sceptile - Charizard wins easy
Mega Swampert - Charizard easy
I guess it's very good 1v1. I remember winning quite a few battles with in on 1v1 mode in showdown. I still don't think it's that useful competitively, unless you're really good at building your team around it to set it up to sweep late game.
If someone wants to continue the list:
http://bulbapedia.bulbagarden.net/wiki/Mega_Evolution