finally managed to level 30 pets to 25 (mostly by capturing 30 random lvl 25 pets and repeatedly beating the Pandaren Earth Spirit while having Pandaren Spirit Tamer
quest in my log) and got into the Celestial Tournament.
Mechanics wise, the tournament is a scenario. It's an instanced version of the Timeless Isle w/ 3 tamers. The rules are you have to beat the three tamers w/ the pets in your stable w/ no bandages or revives between fights. if you lose, you can create a new team and try again, with the obvious limit being how many pets you have. If you're out of teams, you can leave the scenario, revive, and try again.
first impression trying to beat these tamers w/ the pets i have is that they're tough but would be very doable w/ the right pets and some thought. I can see folks who have the bare minimum of 30 pets struggling though.
so far, it looks like fun stuff.
As far as the pet rewards go for battling, I have Chi-Chi, and it's ... good for pve, but i'm doubtful about pve. it's got great avoidance, but the first slot abilities both require speed, and my build only has 273.