Hi Justin,
I've had some experience with BrightTalk and the jury is out for me. My last company just used the platform to post videos (webinar recordings and the like) - it was not used as a replacement for their use of GoToWebinar in that case.
Lead generation from the actual BrightTalk community was slow, and quality was (in my opinion) low. Also, BrightTalk does not support integration with Marketo unless you're subscribing to whatever their premium level is (we had their standard subscription, which, at least at the time, did not integrate).
Overall I think the potential is there for it to be a good lead gen tool, given the closed community and content-sharing focus. But based on my experience with it, it's not a GoToWebinar replacement (as far as hosting webinars...unless that was a feature we simply didn't have access to, per our subscription), and I recall it being quite expensive for what we were getting out of it, as well as the need to be on the premium package to use the API