Being a former Palm (Sony Clie) user for many years, I came to rely on HandyShopper. HandyShopper itself is an application for the PDA, but HS2Edit is a desktop application (runs in Microsoft Excel) which is used for inputting/modifying the HandyShopper databases.
I tried ListPro, and while it is a nice program, I couldn't be as intuitive w/ it (probably stemming from the fact that I am an Excel geek).
FWIW, I wouldn't even step into the darkside of PPCs until HandyShopper and HS2Edit were modified for PPC use. ;)
You can get HS2Edit from PalmGear (you might be prompted for a Palm PDA device name, so just enter a default and keep going).
For help on HS2Edit:
http://tech.groups.yahoo.com/group/HS2Edit/
For help on HandyShopper:
http://tech.groups.yahoo.com/group/handyshopper/
Hope this helps some! Both of the forums I listed have very good info, especially for those on PPCs now. :)