so patch 5.2 brought us xp for our pre-90 toons (pre-89, actually). i rushed out and checked it out with a level 85 in the valley of the four winds, and was pleasantly surprised at how much i was getting. about 90-100k per fight, depending on the level of my opponent. so, 130 fights or so would be enough to gain a whole level - easily doable!
then i started reading that in lower level zones, things seemed rather amiss. people were gaining absurdly high levels of xp, far, far more than could be obtained via questing in the same time. and two days into the patch blizzard announced that that was not what they had intended, and that there would be a hotfix real soon. that night i checked it out, purely out of a selfless desire to have data to provide to my readers. sure enough, without leaving stormwind, i was able to get my mage from 34 to 52 in just a few hours. got several pets up to level 6 along the way. it was fun but, well, kind of silly.
as promised, the servers went down friday morning, and when they came back up the party was over. character xp now scaled with the zone you were battling in, as well as with character level and the difference in levels between your pets and the opponent. so what exactly did that work out to? here is what i've found so far:
in the valley of the four winds, a level 85 toon with a standard leveling team (two 25s and a baby) now gets 80-90k xp. a little less than before, but only slightly. a level 80 toon in mount hyjal using the same leveling team got about 20k per battle, which is fairly proportional to the amount needed level up (maybe 150 fights per level). using low-level pets in orgrimmar with a level 85 toon, however, yielded zero xp.
based on what i've seen and read then, leveling via pet battles is viable if you like doing pet battles. i did about 300 over the weekend, got my 85 up to 86, my 80 up to 81, four pets to 25, and captured three rares for species that i had previously just had poors or commons. oh, and got a flawless critter battle-stone. as much daily questing as i'm doing on my 90s now, anything that helps out with lowering the quests my leveling toons have to do is a win for me. as always, your mileage may vary.