I grabbed Safari 4 over the weekend. I had read about a bunch of the new features, but the one thing I didn't get from those reviews was just how fast and responsive it feels. This morning I launched Firefox by habit, remembered I had the new Safari so I launched that as well. Safari was open several seconds before FF, even though it was launched several seconds later. Also, GMail feels faster, from clicking a link to having something happen, it's definitely better. I'll have to look for some benchmarks to confirm my hunch.
I'm having GMail problems. That's going to prevent me from continuing to use Safar 4 for a while. I'll be trying to send an email and only getting server errors, or it sits there and doesn't do anything at all. Oh well, was fun while it lasted. Can't wait to try a later build.