I have definitely noticed something "different" (like when the earth stopped moving for a few seconds) about someone at first glance on like 3 different occasions and was kind of blown away...I dated 2/3 of them, one long-term, one shorter term, and the other was not available but he was a good friend for a long time.
I believe you can have an instant connection with someone, but I think it takes longer for real love to develop.