I went last year, and just went this past sunday and it was amazing!! Considering I don't give wonderland all that much credit the rest of the year, they really pull off fearfest amazingly well.
The theming is amazing, and if you get lucky like us, zero lines. Maybe people forgot it was open on sinday, but when we went every single ride was walk on all night. We literally (no exagreration) did every single ride that was open at least once. And all the hanuted house once as well. (there are 6 or 7 of them)
The haunted houses are done really well, and the scarers are all unique, so just when you think you have them figured out, they change it up on you.
And those who really like rides, doing them with all the lights off is a whole new experience as well.
If your going to go, go as soon as you can. Lsst year when I went it was a lot closer to halloween, and the lines were long. We only got to do a few hanuted houses, and they didn't have the same feel because there were people in front and behind us. But this time because the crowds were low they kept gaps between groups, so you really got the "how do we get out of here" feeling.