Fox, it's because you know how to counter Scorpion and Smoke. If there were more of these Sindel pucks showing up, you'd know all about her just as everyone knows all about Scorpion and Smoke's tricks. I have used her spammingly in a few matches, I admit, sometimes I just like to mix it up and choose different chars and half the time I don't even know what I'm doing with her. So it's kind of par for the course that's I'm going to be spamming a bit trying to figure out what works sometimes. Anyway, what I think is the case is that compared to Scorpion and Smoke her special moves are faster and harder to respond to. You also can't bait it out like you do with Scorpion or Smoke because of her mixups. But that just means that the spammer must at least have some skill to use her like that because he's actually reacting quickly and looking for holes in your defense.
As for me, I'm going to learn her like she's supposed to be played at some point, for now it looks like she's quite difficult to get into. Spacing is very important, as are mixups. Knowing when/how to use the levitate, imo, is key to using her. Sure, you won't have much use out of it in half the match ups, but you can still use it to confuse someone. It opens up so much more options, it's like being a 3D character fighting 2D enemies since you get a completely different way to play with the character.
As for Sindel played by a good character -- Aside from having alot of zoning capabilities, her close-medium game is hard to understand, and hard to figure out how to defend against. She can come at you from many different angles. her Stepup and Hair Flip moves are amazing, especially when you realise they are mixups from medium range. One hits overhead, the other low. Both have longer range than you're going to expect. The only problem is that Stepup seems a bit unreliable when it meets other special moves, kind of like Kitana's Square Boost. It's hard to tell which of them is going to hit, but Stepup does seem to have a slow startup. If the other guy uses a special move with enough range and speed, it's going to hit. Otherwise it will probably wiff and then you hit them. Another issue is that when the opponent jumps over you as you're doing the step-up, you'll be left doing a very slow move in the wrong direction and eating a combo. Her pokes seem really fast to me, very reliable. So I'm not exactly a Sindel expert or anything, but she seems like she has alot of options and potential.
I'll give her a 7.5.