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As far as looks are concerned, the new release is almost identical to Adobe Reader 8 though it does load slightly faster and is quite nicely integrated with Acrobat.com and Buzzword (read Acrobat.com Review).
That means you can now upload documents & photos to the web from Adobe Reader or even create new PDF files using the PDF writing engine of Acrobat.com.
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Well, if you use any of the acrobat.com services or exchange PDFs created in Adobe Acrobat 9, this is a must have upgrade.
Others may want to download the ~34 MB installer of Adobe Reader 9 if they are looking for some speed improvements.