Mobipocket reader was the first "book reading" software I tried when I first got my clie (seeing it's the supported format on Baen's website).
I thought it was pretty good. Good font support for Clie's, and reasonably fast. Then I discovered iSilo, and never looked back. If a title I really want to read is only available in mobi format, well, that's why I still keep it on my memory stick. iSilo always lives is ram....
Regards,
Craig.