I work for a large Fortune 500 Firm and we were told to hold off on upgrading. It even says as much when I log onto my email through Good for Enterprise.
My firm said they were ready by the end of the week, but Good said they need more time. Supposedly you WILL lose access if you upgrade ahead of time, but probably best to check with your company IT.
Good works fine with iOS 5, I've been using it since Beta 2.
My company just sent an email out to all our iPhone/Good users telling them that the app was OK'd for use with iOS 5 and worse case you would need to enter a new PIN