I've registered, but I didnt really build a strategic team, which I hear is very important for double battles. Too bad most legendaries are not allowed, as most of my strategic teams involve 2 or 3.
I cant even practise, finding players in GBU is so difficult in my location. Once when I did get in, I was placed against a cheat who had all shiny and probably all 31 IV pokemon.
For shiny and 31 IV, he was playing with legit pokemon as IV+shiny is incredibly rare, but it can still happen. Though its very likely he cheated. OF course, you can abuse RNG to get these pokemon.