I did it at POR at the end of April. My original room request made over the phone when I made the reservation was granted. The regular check in line was about 15 - 20 people long with 4 CM's working the line. The online check in line had 3 CM's and 0 people in line! It worked GREAT! That being said, not enough info has been reported yet. I have a feeling the reports you will get on online check in will be similar to Disney bus transportation - some like it, others hate it.
I have read that SSR had problems during the initial days it was offered. I haven't read any lately, though.