I just installed iDroid(Android 2.2) on my iPhone 3G. I must note that this is my first encounter with Android 2.
Beside the disadvantages given by the fact that it's not a native environment(camera not working, 3G internet doesn't seem to be working, buttons seems a bit strange, no sound support for alerts) there is one more issue: Android Market is missing.
As a pro i can say that sync with Gmail is very easy and works pretty good. The emails are rendered extremely fast :o
As a final conclusion i think it works pretty good on a native system, but before i get one i'll make one more test on a HTC Diamond that is somewhere in the house... relaxing...
If you want to try iDroid by your self please note that you'll need to have the iphone jailbreaked - http://www.redmondpie.com/
How to install iDroid/Android on your iphone - http://www.redmondpie.com/how-to-dual-boot-android-2.2.1-froyo-with-ios-on-iphone-3g-2g-complete-step-by-step-video-guide/