The P800 was the first Sony Ericsson-branded smartphone and was feature rich: built-in camera, a pen-operated touch-screen, media player, Web browser, organiser functions, PC synchronisation, GPRS, MMS, Bluetooth and Java. 3rd party apps had started appearing on handango. It had a great form factor as far as smartphones go. And it was curvy in a cute way. All subsequent models from Sony Ericsson went business like steel greyesque straightlined ones.
I used it for a lot of stuff. I got GPRS enabled and even used to surf the web. And Yahoo IM though Agile Messenger was a commonly used app. I heavily used this till mid 2005. In between at one time the screen broke (I'd dropped it) and I had to give it for an expensive repair (cost about 200$). I still have this phone as a back-up and I'd last used this in 2009!
Overall, I think this was a phone much ahead of it's time and I thoroughly enjoyed having it. Many phones I had since then were lot less impressive.
The thing I like the most? The rotating jog dial on the left of the phone. This made it possible to scroll and click on this phone even with one hand. I just love this feature. I guess it's a SONY proprietary design (the j70 had it too) and I miss this on most other cellphones.