I think the chat is awesome on mobile. I especially like the look of the proactive trigger notification on mobile devices.
However, I think it's a shame that I have no ability to reposition this widget on mobile. Part of delivering a good UX is in delivering as uniform an experience as possible for users across all platforms. If I am able to move my live chat widget to the left side of the screen on the desktop, it should be allowed to reposition it to the left on mobile as well!
This is not the case on my current site, but what if the chat icon was blocking an element in its current position? Having no ability to move it would greatly dissuade me from continuing to use Zendesk Chat - especially considering how this could affect conversions. With limited screen real estate on mobile, positioning is everything for conversion rate optimization.