She can be amazing when people use her right. I've seen a Syndra blow her ulti on the tank out in front doing zero damage (well... relative) but that split second later when she stunned the whole team we ran in and destroyed everyone.
She rarely has trouble winning a lane (may need a gank or two depending on opposition) if you can manage her mana right.
Her main issues really are just she's so god awful SLOW and her tells and the delays can be the death of you. Also people need to learn to use her scatter properly. It's good for peeling someone off of her, but when she throws them out of range of everyone/lets them escape she can sometimes get herself killed when they immediately reposition out of range and turn around and blast her.
Her main issues seem to be in team fights. Her setups, when they work, will dominate. She's ridiculously position sensitive and needs a LOT of micromanaging to work effectively though. With her newfound minor damage/ratio/mana buffs, I've played two games with her. One, someone took her top lane so I'm not even going to TOUCH that, as you can all guess how that went, the other was Syndra vs Vlad. She held her own, for the most part, in her lane and as stated before pulled off an AMAZING group stun.
A Syndra player that can keep a level head when everything goes crazy in a teamfight, and makes sure not to fight alone will dominate. Her only real troubles is if she gets cought alone because she doesn't have the best kit for a hard escape or taking on two at once.
In the end I would still prefer someone rocking a Zyra or someone else with easier to land snares or better AoE presence, but with the patch and seeing some people using her kit effectively, I no longer am hit with that wave of pessimism on the outcome of a match.
TL;DR: Great in lane, suffers in teamfights a bit without a team or individual skill to let her work her little setups and help you dominate. Patch has helped amazingly well. Her group stuns after her ulti will win games. Suffers from lack of speed and more consistent snares or AoE, but makes up for it with endless utility.