All of our experiences with online check-in (3 occasions) have been positive. In fact, on each occasion when we arrived to check-in at our hotel, our room was ready for us before 11am.
The only drawback I could see (which we haven't experienced) would be if you were not happy with the room that they prepared for you. For example, if they assigned a ground floor room and you wanted a 2nd or 3rd floor room. I don't know if they can change it for you after you've checked in online.
I hope you make a good experience and have a great time!