Manning looked good his rookie season he just made a lot of bad decisions. I believe Brees was pretty good his first few years too. Both of them showed the ability to dissect defenses with their arms, but had to develop the ability to read the defense.
Henne has shown a gun, but I am not sure about his accuracy. Sometimes it looks like he can thread a needle, then he overthrows a wide open receiver by a lot. I don't think we will have to wait 3 years to see what his physical capabilities are, we should have a pretty good idea by the end of this season. As of right now I don't know if he is good or not.