My dad has an Ipaq that came with Windows Mobile 2003, which we then upgraded to Windows Mobile 2005 when the upgrade disk came from HP. I don't actively use it, but it seems that the PDA has been slightly more unstable since the upgrade. I speculate that if you got it on a PDA new from the store, that you probably wouldn't have these problems, but I don't know. As far as improvements from 2003 to 2005, I didn't really see that much, but neither my dad nor I are PDA gearheads, so there are probably some things there that we just don't see.