Finally decided to give this thing a try this week. Looks like a nice way to kill some time here and there when you are, say, waiting for the coffee to brew, but I have to agree that the rewards are rather underwhelming. I looked around, and found out that the difference between the month rewards for the Platinum League and the Diamond League is two fragments of a Shrine of Knowledge vs. two fragments of a Wishing Well (guess which one is for the Diamond League!). On Global Ranking, the difference in weekly rewards for the #501-#1000 bracket and the #1001-#3000 bracket is 15 medals--almost as many as a Terrace Farm produces every four hours if you tells it to.
And I did run into the same players again and again, once three times in a row after I fought them. I'm not complaining about it--there's definitely an advantage in knowing what units you will be facing, but after a while, yeah, it can get boring.
But I think I'll stick around. At least now I don't have to bother my neighbors when a quest asks me to defeat 20 or 30 units.