Makes it very easy to troubleshoot client issues. We give most of our clients "Editor" access, so their admin UI looks much different from ours. This allows us to easily see what they're looking at.
And switching back is a breeze. (although I think the full-width, site-wide notice is a bit much). It shows up at the top of every page, so if you're doing a lot of testing, it can get a little annoying.
I highly recommend this for developers & site managers. -- I HAVE NOT tried it out on multisite or BuddyPress.