Mostly hate them....which is why I'm making one, lol. I love finding crappy tropes and challenging myself to make them work.
That said, a love triangle is pretty common in real life. For one reason or another people can and do love more than one person at a time because different people do fulfill different emotional (or physical) needs. And different people do react in different ways to this; they might choose, they might try to play both sides, they might go for a menage a trois etc. It's fascinating to see all these reactions.
The problem is how this ends up depicted in media due to lazy writing. If you are setting up one OTP you have to give the readers a convincing reason why this third wheel would threaten the OTP status. However, this is a fine line to tread because hey, if you're going for "tru luv" how true can it be when a hot interloper is enough to break them up? One of my pet peeves is when the introduced rival is actually better at everything than the main love interest, there is a lot of chemistry between the hero/ine and the rival and s/he still gets rejected without a valid justification. Wut??? Or when the main love interest reacts with creepy jealousy and controlling/stalking behavior and that's portrayed as romantic. Like, I love you so much, I'm gonna lock you in my basement to keep you from seeing this other hot bishie...it's tru luv (tm). What the hell, Charles.
The way to make this trope work is if you give every character a fully fledged personality and then base the attraction between all of them on that, not just because "hot dude/chick appears". What traits does any given character find irresistible? Do they find them irresistible enough to abandon everything? Or can they smother this feeling? Whatever their reaction, it cannot be arbitrary, it must be based on personality, upbringing, moral values etc.