The only thing I do not like about iMessage is it as not as unified across platforms as I expected. If a friend sends you a message from his phone, then from his iPad, it shows up as 2 different messages even if you have his information all stored under the same contact. It can just get a little confusing dealing with 2 different messages for the same person.
This could just be an unreal expectation on my part. I guess it makes sense to have it separated by devices so you know where the message is actually coming from. No sense sending a message to a friend's phone if he's messaging you from an iPad. Then again you could just send it to the persons email address and it would go to both devices. Lots of options to think about.
Agreed, that's a pain... but here's a solution.
If you go into Settings > Messages on your phone, you can change Caller ID to your iCloud address. That way, any iMessage sent from your phone will go out via your iCloud, so it will be unified between your phone and iPad. That way, your phone and iPad will both get every message when someone responds to that.