Been using Office 2007 Beta intermittently for about a month. The 'ribbon' system of contextual toolbars and menus is significantly different and does take some getting used to, and it takes a while to learn the new layouts. However the real-time previews of formatting, fonts, etc is very good and I can see myself switching over to 2007 in due course.
I installed Office 2007 on a 'virgin' system and would not advise trying to install it in parallel with Office 2003, Office XP or whatever. Apart from anything else, I'm pretty sure that Office 2007 would 'steal' the file associations for .doc .xls .ppt et al