I have the same internal mic issue(s) described in the posts / replies. Siri worked perfectly on my iPhone 6 running 8.0 when I activated it. At some point it stopped working / understanding me but not sure exactly when. I didn't upgrade to 8.0.1, upgraded to 8.0.2 (this is when I first noticed the issue but not sure if it is related to 8.0.2) and am now running 8.1. I had two bluetooth connections, Uconnect with a Jeep (wrangler and cherokee).
I have tried the suggestions related to bluetooth....initiating siri interaction in both Jeeps (all works great by the way) then disconnecting bluetooth and resetting phone (iPhone 6 running 8.1) and it didn't solve anything. I currently can only use siri with the wired headset.
Side note, I have an iPad Air running 8.1 and everything works perfect. Its never been connected to bluetooth, so maybe that is the issue?
Hope this is figured out and solved soon.