Probably specific to my audience, but in all of the tests I've ran optimising forms for conversion - Using sliders have always had a negative impact.
Using conditional logic for questions and breaking into steps have given us our biggest wins in conversion terms.
Interesting. May I ask what type of forms you're talking about and what was the winning alternative?
At the moment I work closely with a service designed to help people switch energy to cheaper providers.
Some of questions users need to answer in order to complete their task are quite complex.
We've replaced sliders that were used to enter usage details with text fields and seen positive results.