I've always had GREAT luck with online check-in, especially with room requests. Our past two stays (CSR and WL# we not only got our requests granted, but both times we were upgraded too! I think they will definitely try to have your room ready, but there are just a lot of factors that affect that, so it's hit or miss and I don't think online check in makes a difference other than they can "plan" to expect you early #for whatever that's worth#. When we stayed at CSR, we did online check-in and arrived around 11 or 12 and our #upgraded!# room was ready, so we just picked up our package and headed straight to the room to change for the pool!
Another advantage is that the online check in is a time-saver - I've always found the online check-in line to be much shorter than the regular line. I really just don't see a disadvantage.