2590.While I like Firefox 4 muchly, I'll be going back to Firefox 3.6 for now. As soon as the PDF plugin maker makes that plugin okay for FF4, I'll be upgrading. Also the DIVX plugin won't work with it … for now.
If you don't depend on those plugins or your workflow doesn't demand that they be there, I say go ahead and upgrade. But it's nice to be able to view PDFs in the browser, which is the biggest flaw in an otherwise fabulous browser.