I love reading on my O2 mini, using the Microsoft Reader. In fact I read mostly on it nowadays, as a supplementary to my printed documents. It’s a handy little device, though there is much to complain about the reader software itself (lousy interface, bad bookmarking etc), it’s the best and most popular in the market now. Adobe Reader sadly sucks so badly on the Pocket PC I don’t even bother to read anything on it anymore.
Here’s a tip if you plan to read using the Microsoft Reader — you can convert your documents from Microsoft Word! Just download the Reader plugin at http://www.microsoft.com/reader/download_rmr.asp and install it. When you save the document in Word you will have an option to convert it a .lit format readable on MS Reader. This is how I read most of my documents and articles.
For PDF files, you can do a conversion from PDF to Word or TXT or RTF (using any tool in the market — there’s plenty) and do the same thing.
Another tip — if you like classic books check out the Gutenberg Project, if you like Science Fiction or Fantasy books, a great collection of books is offered by Baen, for free! (I kid you not). There’s plenty of free and good stuff out there if you take some time to sniff around.
Drop me a note if ever you find anything interesting to share!