Format anxiety
I am a long time Mac user and have been through many transitions during the years. The 68k to PowerPC processor transition was allmost unnoticable for me as a user. When I switched from my Performa 450 with System 7.6.1 to an iBook Clamshell in 2002, the iBook ran all my programs just fine, even if many of them were from 1994 and for 68k processors. Everything continued to work well even after upgrading the Clamshell from Mac OS 9.0.4 to 9.2.2. No backwards compatibility problems at all.
The transition to Intel was harder. My iBook died when I stumbled in the power cord in 2006, so I bought myself a MacBook for Christmas. (I was able to ressurect the iBook in 2008, though.) I didn't use OS X on the iBook, so that was a new user interface to master and in addition to that, none of my old programs could be run on the MacBook since Intel Macs didn't support Classic Mode even if I was running Mac OS X 10.4 "Tiger". Luckily Apple supplied some great free programs in the iLife suite and I soon downloaded OpenOffice and some alternative browsers. I got a copy of AppleWorks 6 and that helped me slightly further (although not as much as anticipated, since it would not read my old ClarisWorks 2.1 documents, only my quite new AppleWorks 5 documents).
In 2009, I upgraded my MacBook from Tiger to Snow Leopard, wich was a huge improvement even if it was a bit buggy in the start (this is the only time I have been an early adaptor of a new Mac OS) and in 2010 my MacBook died and I bought a MacBook Pro. (Later I was able to ressurect my MacBook's hard drive, so even if it lacks a battery and optical drive, it works. I use it for wireless backup and for ripping DVDs with an external optical drive.) I also bought Finale PrintMusic, a light version of the Finale music notational program, since I was a heavy user of Finale from 1989 untill 2006, but OS X broke it for me at a time when I couldn't afford bying a new version for OS X.
Yesteryear, I got iWorks from my mother and took a day to convert all my old ClarisWorks 2.1 documents to AppleWorks 5 on the iBook, convert all of those with AppleWorks 6 on my MacBook Pro to AppleWors 6 documents and convert those further to Numbers and Pages, in addition to my AppleWorks 6 documents. I find it quite ridiculous that Pages will read Microsoft Word files in all types of legacy formats, but not AppleWorks and ClarisWorks documents unless they are AppleWorks 6. It is sort of Apple's way of giving the finger to their own long time customers at the same time as they are giving a helping had to long time Microsoft users.
AppleWorks 6 also had much better support for legacy Microsoft formats than legacy Claris/Apple formats. Even if Apple is constantly going forward and trying to promote itself as the company of innovation, most people actually like to be able to open their own documents, even if they were made some years ago. Being able to open old documents made with Microsoft software, but not with Claris/Apple software is totally insane if Apple want people to use and trust their software. It gets even more ridicolous when the ability to open your documents in older versions of the same program is not available either, since the newer Macs are unable to open pre OS X applications, so if you don't have an old Mac lying around, you are totally unable to convert your old documents to newer formats. If you go from a 386 IBM clone with Windows 3.11 and Microsoft Office to a MacBook Pro with Mac OS X 10.6 "Snow Leopard", then iWorks will import all your documents just fine. But iWorks won't open your ClarisWorks 5 documents made on a PowerPC Mac in 2003.
After these experiences, I am very cautious about saving my documents in Apple's own formats. Even if Microsoft has also changed document formats many times to force customers to move to newer versions of their software, they still make sure that their new software will open their own legacy formats, so their own users don't loose the ability to open their documents produced in earlier versions of the same software. Any software company with respect for their users would do this when moving to new document formats.
I am not really willing to bet my documents on Apple's continued support for their own formats, so I have converted most of my text documents where formatting is not important into Rich Text Format (RTF). Rich Text Format has been with us since the dawn of time and is still a widely supported format that includes support for bold, italics and other simple formatting, as well as less problems with using letters outside the English alphabet across platforms and text encodings, wich is quite important if most of your documents are not in English. I have yet to come across a word processor unable to open RTF documents.
For documents with page formatting, like term papers and such, I have made PDFs of them in additon to converting them to open document format (.odt) wich is usable in LibreOffice, OpenOffice, NeoOffice, AbiWord and most other open source word processors. I had to convert to Microsoft Word format first in Pages and AppleWorks 6 and then open those word doceuments in NeoOffice to convert them to open document format.
I have also converted all my old ClarisWorks spreadsheets into Numbers documents quite recently, but I am now thinking that I will convert those as well to open document format (again I will be exporting to MS Excel and then use "Save as.." in NeoOffice. I converted my old MacPaint, MacDraw and ClarisWorks draw and paint documents to PDFs a long time ago, so all of my old concert posters and such are still with me.
There are many good reasons to use the open document formats. First, they are widely supported by a lot of software that is under active development by people in the open source movement. Even if it is not such an old and widely supported format as RTF, more and more programs are including support for opening and saving in open document format. As I am not really willing to buy proprietary software to do things free software does just as well, it seems a good choice both for my wallett and for not becoming to dependent on the whims of large companies like Apple and Microsoft.
Another reason to use open document format is that whatever operating system you are using, there is software easily available that will open these documents. This means that you can mail these documents to anybody, and they will be able to read them easily, and if you in the future should decide to change operating system, there is no need to convert your documents, since software that read these files are available for all the major operating systems. With the direction of both Mac OS and Windows going towards forcing the otherwise brilliant tablet and smart phone GUIs onto desktop and laptop users, with more or less lock-in to their own app stores, and less and less availability of the file system, I am getting more inclined towards changing my operating system. I have started exploring Linux distributions in virtual machines with the VirtualBox, but that is another story.