I’m running a Mac Mini with Windows 8 (Bootcamp) as of the moment, with the HD 4000. I’m not noticing any serious issues, though it can become a little slow when handling heavy scenes. In order to solve this, I simply turn down the graphical quality and disable the lighting.
I believe certain ‘low-hanging fruit’ as Epic’s put it, will be improved in the forthcoming 4.3 release, so hopefully that’ll help speed things up more! 
Especially as they’ve mentioned improving performance on laptops and the like. Fingers crossed. Though, it’s more than workable for the time being!