Like anything else, it depends on how well the author sells it. I know of quite a few real-life occurrences of love at first sight (my parents included) on one hand; on the other, "It really happens all the time!" isn't a get-out-of-suspension-of-disbelief-free card. If the author wants the readers to believe the characters are instantly in love, they must convince them. Lots of people see lots of other people they find attractive all the time; a love-at-first-sight scenario absolutely must show us, the readers, why this time turns out different from every cute barista, cashier, and exchange student the MC randomly encounters in a day. It's only bad if it's badly done.
Because, IMO, as another person who's not all that big on romance most of the time, long courtships aren't any more convincing when they end with the readers still seeing no reason the characters are together beyond "the author says so."
ETA: For a good example of instalove, I'd bring up Deep Into the Heart of a Rose's romance between Miss Ashley and the Wicked Tinker. She's someone who can both literally and metaphorically see things other people can't, and he's a brilliant artist who has been unjustly made a pariah. She seeks him out after seeing an example of his work, and next the reader encounters them, they're the beta couple... but with their backgrounds, and the book's general dreamlike tone, that made sense to me.
(Fair disclosure, the story I'm writing plays with a few tropes, and one is insta-love. The hero and his LI kiss within ten minutes of meeting without even knowing each others' names... because they share a flaw of being extremely impulsive, and by the time the reader finds out about the kiss, they'll have seen both characters take far more reckless actions on the spur of the moment. However, the kiss isn't the point they actually fall in love. That takes the whole book to happen).