The update is not mature. I can confirm the reported issue "enigmail 2.0 breaks s/mime". I am no longer able to sign with s/mime by default for non-pgp e-mails. Is this no longer possible? The new settings for signing and/or encryption look quite confusing even for experts.
For all non-pgp receivers I normally sign with s/mime and add my public pgp key as attachment. So every receiver has the chance to trust my e-mail and also to reply encrypted.
This review is for a previous version of the add-on (2.0).