It works really well and no problems with multisite.

I think this would be particularly useful to anyone running a lot of sites and multisite installations, jumping quickly between dashboards - a quick glance tells you that the site is displaying correctly without having to separately visit the homepage.

It would be even more useful if the site preview widget could appear on the Manage Themes page, it would make testing different themes a lot quicker.

Honestly, I'm beginning to think that this should be a standard part of the WordPress dashboard.