Tons of applications, unearthly-long battery life, perfect integration with your desktop computer, compactness, reliability. Better things are coming out with the same OS, but that means the IIIx is getting cheaper and cheaper, and no less remarkable. No color, not for memory-hogs, soon to be beaten out by better devices on the same platform. Lot's of memory. Lots of memory, small and portable, many uses.
Batteries die kinda quick.
Lacks in processing speed. Lots of software, stable, great battery life. The built in software could use a little tweaking. The III series is becoming a thing of the past. It's all about the cradle.