I run MS Office 2007 on XP Pro and Vista boxes. I run Ubuntu 9.04 on the the same machine as Vista.
I installed Ubuntu & OpenOffice (same distro) because I am frustrated by MS constantly fixing what ain't broke (OSs and apps).
OpenOffice compares to MS Office - features and functions - in a word, "poorly." There are things MS Office and OS-related aps (e.g., Outlook/Outlook Express/MS Mail) simply do better.
There are apps under MS OSs that have no comparable (free) Linux application - example: desktop publishing.
The load times for apps under either OS seems about the same, and OpenOffice.org apps convert MS apps quickly and, from what I've seen, cleanly. MS can't convert OO.org, but OO.org CAN save files in MS format (albeit perhaps not the most recent).
In my daily life I am a heavy word processer user and a light user of spreadsheet and presentation (e.g., PowerPoint, Presentation). I also use some of the photo apps (the nod there goes to Ubuntu). I am not a gamer.
I cannot totally abandon MS products, but I am more-and-more using Linux' free apps and utilities; they just take some "getting used to."
Again, the switch primarily is do to MS mucking about with the UIs.
UserAgent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:188.8.131.52) Gecko/2009080315 Ubuntu/9.04 (jaunty) Firefox/3.0.13