I'm for spaying/nuetering but I personally have had problems with a couple of animals. One developed myoclonus shortly after surgery (most likely he had distemper as a pup and made it [I adopted him]). I have no idea if the stress of the surgery actually had triggered it or if it was just a coincidence. I had to put him down because he couldn't pant, eat, or drink... poor baby.
The second was another rescue who must of been used as bait (missing/ broken teeth, broken upper jaw and ribs) or tortured. She was a sweet dog but after getting spayed, she became extremely dog aggressive.
But those are the only incidents that were negative. The rest were completely positive, but there is always a few animals (same with people) that will have negative effects after any kind of surgery.