We pushed out IE9 with WSUS after testing and all seemed fine.
One major website we use for management turns out to be incompatible and portions of the site don't work well (if at all) under IE9 - even in compatibility mode!
The company is updating the site base (hosted web solution/package) eventually with new features & IE9 support but for now we have to (shudder) roll back to IE8 on all our Windows 7 & Vista machines. Apart from going to every PC manually, is there a quicker way?
Also the IE9 rollout for WSUS (although manually installed worked fine) causes IE9 in Vista to immediately crash (regarless of plugins, etc). More reason to boot it off!