I've actually been forcing myself to use the stock browser lately just to put it through its paces and it's basically acceptable. Terrible by comparison to Android and poor compared to iOS, but serviceable.
The biggest issues are the jerky scrolling, the fact that you have to wait for pages to load completely before scrolling and the fact that it still periodically freezes on complex sites. Something about Engadget, for example, absolutely destroys the browser. In terms of rendering pages, it does so faithfully. It's actually really good in this respect. The pinch-zoom and scrolling are very decent once everything's loaded.
If Opera were to update Mobile 10 with just pinch-zoom for S^3 it would obsolete the stock browser instantly. As it is, pinch-zoom is actually enough of a convenience when surfing that it's worth putting up with the browser's jerky scrolling and performance.
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