Although I have not checked how powerful they are, I do know that trading makes them useless. Anytime a pokemon is stored in a box or traded, its happiness goes back to neutral.
As for knowing when its friendship/hatred is maxed out, some items can help you out there. Berries #21-26 will increase happiness but decrease effort values in a stat. I also remember there are some herbs that are very effective at very cheap prices because they make your pokemon dislike you. Yeah, they taste just that bad. I know you can find the bad ones in Colosseum, but I don't know where you can find them elsewhere (probably can, just don't wanna look it up). When they won't increase or decrease anymore, the game will say so. Frustration is harder to do, though, because simply walking with the pokemon increases friendship.