So, I've played two games with it so far (Nidalee and Fizz)
I can't really give that much feedback from the first game since it was a 3v5 for quite some time.
In the second game as Fizz I've had pretty good success, but some problems were that my clear felt kind of slow compared to fizz with runeglaive and I've had more mana problems than with runeglaive, since before I could get 3x8% of my missing mana refunded, while with runic echoes I mostly only get 1 proc per camp and since it's at the start of the camp, it's not as much as I'd get if it was at the end of the camp since I of course used spells, costing mana.
Some suggestions for the Mana Sustain and Clear would be:
1. Make Runic Echoes recharge faster when jungling (When in combat with monsters or something like that)
1. Increase it's damage vs. small monsters aswell
2. Change the mana refund to 18% **max** mana