There are a lot of good reasons to use the latest version, but the main one for me is that it is the only version that works with MS Office 2010. The other compelling reasons would be ease of use, cleaner screen and faster performance.
Adobe’s feature comparison between versions 7, 8, 9 and 10
Acrobat X reviews:
eWeek review: lists 10 reasons in a slide show, some of which are part of Acrobat 9; most compelling slide for me was that the file size could be smaller without losing quality
Notebook review: slick interface, faster performance, improved interface with Word and Excel
Creative Pro review: Actions and the Action Wizard, improved customization for PDF Portfolios, simplified user interface that promotes efficiency.
