I can only describe it from my own experience:
I had love at first sight happen to me. I was 17, I did not know him, I just saw him across the room and just knew he was THE ONE!
From my perspective now I would rather say he immediately struck a chord in me. It took us a year to actually get together, and then we did not have much time together, because he had a fatal accident. It would be easy to romanticise this, but my thoughts are actually very different:
I think it is very possible to fall in love at first sight. However, I don’t think it has anything to do with the quality of the relationship. To be honest ours was really exhausting, and pretty unhealthy. This is not only because we were so young and immature, but maybe because the chord was actually a trigger.
It’s hard to tell now, because we never had the chance to get real and work it out, but I do believe I had much better and deeper relationships after that.
My therapist said something she had learned in her years of work (with a twinkle in her eye): If you meet someone, and you get this really special feeling, and you think there is something deeply resonating in you with that person, this is what you do: You RUN! 😉

Maybe you can compare falling in love with starting a fire: Some start easy, some take more time and effort to start, but the quality and duration of the fire is up to the material itself. A quick start can mean that the wood is very well dried and just perfect, but it can also mean that it will just flare away. A start that takes time and effort might make for a really long, good fire because the materials have a lot of substance that just needs some time to reach the right temperature – but it can also mean that the wood is just too wet to burn and you’ll just get a lot of smoke.

In my opinion every relationship is different, and even though instant attraction is nice, there is much much more to making it actually work. (I used the word actually a lot… guys you gotta correct my English if you notice something please…)