For me, as a (power) user/developer with a decent understanding of what things on the device use how much power, I seem to get between 6-8 hours of heavy usage.

Higher than some because of tweaks rooted in knowledge of how the device uses power (lower backlight, push email, always 3g or always 2g depending on location/situation rather than dual mode)

Lower than some due to high usage and games.

I agree that Constant/Persistant data transfer (no matter how small) are the bane of the devices power source.

The device seems quite good about going into idle/low power states when possible (aside from the potential wifi bug discussed elsewhere on this forum)

But if you have things constantly transferring data, your power efficiency will suck. 3G, and the power saving behavior of the WiFi radio on the device are oriented towards burst traffic with periods of inactivity between (web browsing), not slow steady streams of data.