Love triangles can be nice (though I'm not too fond of them myself) when there is an actual chance for the two "contenders" to "win", so to speak.
Reason why the one in Sakura Trick was so crappy was mostly because Haruka and Yuu were already pretty much together way before Mitsuki came in the picture, so yeah, it felt totally forced and useless as there was just no chance for her to get Haruka without completely messing up the whole story.
It wasn't just forced, it even went so far as to make me completely hate Mitsuki as a character. Trying to steal someone from someone is bad enough, but when that person is also your own sister? Dick move.
The problem with triangles to me, is that no matter who's chosen, there's always someone left behind. So regardless of the outcome, it always ends up un-satisfying. Unless one of the characters is a bitch, of course. I know some people like Mitsuki, but to be honest, I don't, that's my personal opinion on the matter (I'm entitled to one)! :D