Eh, might as well voice my eccentric tastes for a Gmail invite. ;P
Personally I use QT/Opie Reader on my SL-5600 Zaurus (Linux-based; almost a pocket computer
), with Plucker as the main format I use.
Why? Well, there are many books available in the Plucker format (through several sites, including
http://manybooks.net), as well as being able to take HTML news feeds (i.e. Slashdot, etc.), and turn them into nice sized packages for reading. It's open source (for a FOSS [Free/Open Source Software] geek like myself, that's a real bonus), and has no DRM/copy protection on it, as there are no DRM-able readers for the Zaurus.
As for why QT/Opie Reader as the viewer... Well, it's simply the best one for the Zaurus, with only a few others available.