Installed the update. The installer says you can't skip the library update "or some instruments will not sound correct". With 3 hours remaining on the library download, I went ahead and installed the executable update anyway.
Windows once again complained that I was putting my computer at risk by running the installer. Sheesh.
The installer first runs the authorization manager installer, then the custom shop installer (I was hoping there'd be an option to skip that). Installation took only a few seconds. Think we're done? Not so fast - on first load, it rebuilds the instruments database, perhaps on the presumption that I've followed instructions and added the new instruments. This takes a few minutes, so I have a cup of tea. Upon completion, I was relieved that it didn't make me go through online authorization again.
The "improved browser" claim eludes me. All I see is that there is now an icon next to the top-level tree list item. And I have the ability to add a barely-visible grey star next to individual instruments to flag my favorites. No other enhancements are obvious, at least at first glance. I do notice that it's now rather sluggish when switching between views, but it's only a 2-second delay so I'm not concerned. Happy to see my imported libraries and good ol' Miroslav are still there.
In summary, the pluses are: installation was painless, no need to re-scan or re-authorize, and nothing got broken. But don't get your hopes up dub/Rain, it's the same interface, near as I can tell.
Gave the Custom Shop a look (it takes a while to start up, be patient). I've said before that I think this is a brilliant bit of marketing, but I was pretty disappointed by the wares on display there. It's pretty clear the direction IKM is going...Minimal, Hip-hop, Trance, House, EDM, Hardcore, Techno...I feel so old. I'm gonna go put on some J.S. Bach as therapy.