I'm still trying to find my ideal browser, and I've been using Safari, Shiira and Camino mostly.
Safari is quite quick, but I can't access my university's secure e-mail. So that's a pretty big negative (why should I have to open a different browser just to check it, after all?). I also dislike the fact that I can't move the tabs around. (They'll increase tab-functionality in Leopard, I know... Can't wait!
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Camino is nice, renders quick and accurate enough for my tastes. I can access the university e-mail, so that's always good. But, again, the tabs... I like them a bit better than the Safari ones, but they aren't very functional either - can't move them, and when you close a tab, you go to the one to the left of it automatically. I would prefer that when you close a tab, you go to the previously selected one, wherever that may be along the tab bar.
And that's one of Shiira's biggest features, the tabs. They're moveable, when closing you go to the previously selected one, and there's this neat "tab expose", which is always fun. I'm seriously anticipating version 2.0, with the new UI and features. Big minus, though, is that Shiira - too - can't open my university secure e-mail.
So, until Shiira 2.0 rolls around in the completed version (the stable beta does the tabs and allows me to access my university e-mail, but it's still beta and not completely featured), I'll likely be using Camino. If/When I upgrade to Leopard, the choice will probably fall between Safari and Shiira.