Syndra was a pretty big disappointment in regards to her strength. There's no doubt she's a high skillcap champion, but the power of her full combo barely reaches that of the combo on another AP mid which is easier to pull off and far less situational. Not to mention the unpredictability in her skillshots. I've never been less afraid of facing against an AP mid, she's practically a death sentence for the team that has her.
She has an awful 22% win rate. She's a burden more than anything.
HOWEVER, Riot have said that they recognize that she is weaker than intended and are buffing her. Hopefully adding some new mechanic to her Dark Spheres, perhaps pulsing damage as long as they last, that would be nice. An increase in the speed her W throws an orb/minion also needs to be addressed as it is far too slow, you practically have to be on top of the enemy (you're a squishy mage, remember) and then throw the Orb far ahead and hope to god your prediction on their movement path is correct.
If they are going to make a champion with absolutely no built in mobility in a metagame where mobility is king, you have to compensate on the damage or make her utility more reliable, understanding vectors is something that takes practice, but the short range on her E means that you have to be much closer in a fight just to get your stun off. Unless you place your Dark Sphere directly infront of you and fire your E skill, in that case you aren't using your main damage nuke and used mana for two skills but really only having the effect of one. People are preferring to play her as a support champion because she gets outlaned so easily against legitimate AP's.
And I haven't even mentioned her severe mana problems which are somehow worse than the mana problems of Orianna. The thing is, Orianna has high base damage on her W skill, so she can afford to build Chalice of Harmony for the mana regen, but Syndra doesn't have the high base damage so while you now have plenty of mana, you are well behind in power compared to everyone else.
That's enough ranting about Syndra from me though, I think she's really fun but in a competitive game on Summoners Rift, I don't know why you would pick her over other AP's. Maybe she would find a better home in Dominion where fight locations are more predictable (around the Points), so she can set up the battlefield with Dark Orbs better, also Dominion gives you global passive Mana Regen. I could also see her being played on Murder Bridge, as you are literally fighting in a line and theres really no chance of your orbs not being in a good spot. However, champions should be balanced for Summoners Rift, not for the side maps.
In other news, Rengar received buffs in the latest Hotfix: He's a monster now. He was pretty good before as well, but now his damage and ganking capability is super high. Be afraid of bushes.
Vayne also got an extra 5ms on her base, should help her laning and chasing quite a bit.