Fedace.....yoga is awesome. Our bodies are designed to move to maintain musculoskeletal balance and most of us don't move in enough varied ways so we begin to lost that balance. Certain muscles go limp, others get tight, bones are moved out of position, then things don't work and feel like they should.
if the particular yoga class is giving an individual the specific motion their body has been missing, then they'll typically feel a lot better. If like the one person above who didn't feel much, they're probably being given motion their body can already easily accommodate so it doesn't create a functional change.
plantar fascitis is typically a result (in my clinical experience) of a compromised foot strike, which is often the result of hips not where they should be and thus unable to do what they should be doing. Many yoga poses are terrific for mobilizing tight hip joints, so you can certainly try it and see if it positively impacts your pain.