I’m trying it out on my laptop at work to see how I like it (I’m all Mac at home now). In regular use, the speed advantage Safari is supposed to have over Firefox isn’t really noticeable. Most sites I visit render correctly, except for the occasional message in Yahoo! Mail. It has the sort of problems I’d expect rendering ASP.NET web controls, but so does any browser that isn’t IE.
At least for now, I wouldn’t ditch Firefox for it. I might use it more on my Macs though.