I've already sprung the beta viewer on an unsuspecting first time SL user.

He had very little trouble figuring stuff out.

On the other hand, with the old viewers, I would have to spend at least an hour or two getting them started.

Very true. At a meeting today, someone actually referred to it as a "browser".

One of the main complaints that I get/hear is that the SL 1.x interface has a high learning curve.