I just played a game as Eve jungle on PBE with Runic Echoes. With the nerfed mana regen, I thought Eve's mana issues would be really bad, but it was alright, you just have to stay in your passive for a while, and the extra movespeed helps mitigate being conserving with W.
The burst was surprisingly decent when I completed the item, (although it's awkward that Ravage doesn't stack/proc the echo passive) and Q/W can help you build stacks quickly.
My big concern is the somewhat clunky mana regen. You'll find yourself proccing Runic Echoes at the beginning of camps 99% of the time, and with Eve's spammy Q you can sometimes proc it more than once, but I see that as a potential problem for other AP junglers who may not have low cooldowns, as they may be mana starved and not be able to utilize the mana regen proc on large monsters effectively.
Another potential issue is the Echo passive stacking with Thunderlord's Decree. Early on in the game I was bursting pretty hard due to having both. I decided to go hybrid tank towards the end, so I don't know how well an assasination build would work out, but if it does... I feel like the item will be nerfed, even though it seems pretty balanced.
I'm kind of sad, as I thought that maybe if Eve didn't synergize well with Runic Echoes, she'd get compensation for the removal of Runeglaive if she didn't do well with Runic Echoes. She doesn't synergize AS well with Runic Echoes, but she can make decent use of it, and with Runeglaive being overnerfed anyways I doubt she'll be getting buffs now. :/
But on the plus side, if you liked playing AP Assassin Eve and/or Elise, then you can do so more effectively now, until Thunderlord's/ Runic Echoes/ EVE get nerfed.