I have installed Safari 4 on multiple Macs and PCs (both XP & Vista), I have had no issues on either platform, the install went smoothly and the speed boost has been phenomenal. I have been able to load sites on Safari 4 that previously only worked correctly in IE on XP. The fact that I can now access these sites on the Mac is a godsend. Top sites - a nice touch and usuable, Cover flow - eye candy, Tabs on the top - horrendous, it looks unsightly and is not at all intuitive. But the speed, that alone is the dealmaker. 1Password is working (with a minor painless tweak) as is Safari adblock so I am happy. Highly recommended, as with any update, make sure your Time Machine backup is up to date and any potential issue is pretty much a non event.