I do online check in for most visits. I do it so that I can enter my credit card info myself. That way I do not have to give a credit card to the CM at check-in. IMO, that lessens the chance of someone using a skimmer to steal my credit card.
I do NOT do online check-in when we are traveling with guests or children, so that I can set up which bands are allowed to charge when we check in.
I do not do online check-in for the second part of our stay if we are doing a split stay. I also skip any portions that deal with requests. I send MS an email with those shortly after I book.
In our experience, online check in has always taken less time when we arrive than does regular check-in. I know that is not true for everyone.
The success of getting requests seems about the same no matter which method is used. IMO, that mostly depends on what is available on the day we check in and the resort. For example, BWV seems to do more pre-assigning than do the other
DVC resorts so it is more difficult to get a change at check-in. (I never ask for one unless it is offered, though). But that is just my impression and may not be correct. We stay mostly at BWV, but also have stayed at BLT & AKV.
Some have reported that they had to check -in online before making FP+ reservations, but I do not think that is true anymore.
HTH.