I thought it sounded good in theory but from playing games it's really bad. Here's why.
The only use of it is to heal OD in fights, but OD's mana pool tends yoyo up and down and up and down, ranging from 1000 to 2000+ mana. But OD's hp with an aghs and a bkb, and his hp talents, is about 3000. So basically, the only time you get value out of the shard is when you're really low hp, and then you have to pray that at that point, your mana just happens to be on the high side of the yoyo.
So it's really unreliable, and the healing is usually not even that significant unless you manage to get out of the fight, top up your mana to full then do a swap.
The unreliability is an insane dealbreaker too. When essence flux triggers OD can get like an extra 1250 mana (which translates to 1250hp), so a lot is praying to the RNG gods that when you're low hp, your next hit before you die triggers essence flux so you can use the shard healing. If it procs essence flux you get healing and live, if it doesn't, you die.