I recommend you watch the s3 EU qualifiers that went down this weekend. She was picked 22 times and banned 1 time out of 33 total games. Of the 22 games she was in, the team with Zyra won 16 times and lost 6. And she was played both mid and support. That should say something about her viability.
I played Zyra alot in S2. Main mid in ranked and around 75% wintrate- she was kinda OP in mid, yes.
however i never had problems with her as supp. Rly dangerous is her lv 6 burst if she lands her e, but that falls off if she doesnt get gold/assists/kills.
The utility with brushcontrol is rly nice, sure, but i think if you are just cautious against her from 6 on, shes not better than other supps.
Shes just nice and bursty. If she gets some ap she even has even good damage later, but nothing to complain about i thnk.