I've used OO for years as I use a Linux desktop - at work. (I work somewhere were we are free to chose what ever we want. Most colleagues of course, use Windows, but we actually have more Linux/OO users than Macs user!)
It can load/save the older Microsoft formats perfectly fine. There are some times issues with fonts - particularly if you are using Linux as Linux doesn't have the same fonts as Windows - the popular Windows fonts are not free, so the Linux packagers just can't package the fonts into the system.
But these problems are similar to opening the same document on a Mac with Office Mac - the Mac's font metrics are subtly different to the Windows ones and formatting can subtly change as result.
OO will happily open the new MS Office files, but I don't know how well it keeps the formatting or if it can save in that format. I know there are legal issues with Open Source products supporting the new MS formats due to the way MS have licensed and patented aspects of their format.
The biggest thing I've found problems with is macro laden Word and Excel documents. OO's macro interpretor is not nearly as 'compatible' as the document layout parts. I've run into a couple of cases where macro's had been used to make 'forms' in Word and Excel that OO just can't parse properly. I HAD to use MS Office on these files.
And one other time when I had to fill out a table hosted on a Microsoft Sharepoint Server - not only did I have to use IE, Office had to be installed on the machine as well, or the 'online form' didn't work!. (Sharepoint tables use ActiveX to call Excel to do the data entry!)
From my experience, I was say about 90% of the population could do what they need with OO perfectly fine and be none the wiser for doing do.
Accounting types tend to push Excel harder than OOCalc can go. Excel has far better charting for those all important pretty graphics to send to management.
A kinda funny OO story I have is being asked to attend a 'management retreat' as their pet IT person in case something went wrong with the technology. So I arrive, check the projectors, etc and retreat up the back with my Linux laptop, connect to the wireless and start to read my email and work on things remotely. Then the big bosses PA turns up and asks were the computer is. I had expected them to bring their own machines. They didn't, and they hadn't booked any of the loaners. The only machine available was my Linux only laptop.
The big bosses Powerpoint presentation was shown using OOImpress on Linux and he was none the wiser.....
