I've never tried online check-in (we probably will try it before we go in August), but the one thing against it I would add - if you are a first time visitor to Disney (and it looks like you are), then it might not be good for you. Its meant to speed the check-in process along, but if you think you might be at the desk asking a lot of questions, needing an explanation about the room "keys" on your key to the world cards, and a variety of other first timer questions, then you might want to try the regular check-in line.
Thats what I've heard anyhow - maybe someone else can add to this.
Have a great time!