We used online check-in for our November 20, 2010 stay. First of all, the system did not work on the numerous times I tried to check-in at the 10-day out mark. I had to call MS and was told that there was a bug in the system as they were converting reservation to the new system, and that I had to re-enter information that was automatically filled by the system. Then I was checked in!
When we arrived at BCV, the front desk had nothing prepared for our stay and we did not get anything close to our requests, even though we had booked at the 11-month window and put our requests in, plus we had done the online check-in.
So, it did not work for us, did not save us any time, and was frustrating to use. We will try again this year, and hope that the kinks are worked out for our stay.
On the plus side, we had the best stay EVER despite the bumpy checkin, so hey, I am a happy Disney nut

!