As part of the online check-in process they ask for the time you plan to arrive at the resort. According to CMs I've spoken with, they use the arrival time to try and arrange for you to have a room ready when you get to the resort. This is mirrored by my own experiences.
So, if you will be arriving early and want the greatest odds of having a room ready for you, use the online system.
However, the trade-off is that they do not appear to put as much emphasis on any room requests when you use the online system. Again, the focus is on getting you into a room--ANY room--not necessarily one that matches your ideal accommodation.
So to summarize:
* If you place the highest priority on getting into a room immediately, use online check-in.
* If you place the highest priority on having your room requests (view, location, etc) met, then do NOT use the online check-in. You may have to wait longer to get into a room, but you will typically find that the front desk CMs are more willing to work with you during that personal check-in.
The time savings aspect is very hit-or-miss, IMO. Sometimes the packets are printed and ready to pass out. Other times the materials have not been prepared in advance or (like other posters) you end up behind people who still want to ask a dozen questions. :brick: