Online check-in is "marginally good" (in my opinion) if you have nothing to do to your reservation at check-in...
It does save you a few minutes (does nothing with respect to whether your room is available) in that all your paperwork is ready to go...but you could be in an online check-in line as readily as in a "to check in" line...it hasn't changed the wait time to check in at all, for us, in the many times we've used it. It does shorten the time to check in once you're at the front of the line, though--although not by very much.
And it's really not good if you need to do something "special" to your reservation...for example, on those occasions that I use the dining plan (increasingly infrequent), I use Disney Visa reward dollars to pay for it...w/online check-in, the dining plan charges are already on my credit card...so this type of situation means I shouldn't do online check-in.
Or, if I want to move rooms around (e.g., if I really want a particular request (sometimes I care; sometimes I don't), it's better not to do online check-in--that way, if the request hasn't been granted, you have a chance to ask if something can be found, particularly if you're willing to wait for a room that isn't yet cleaned/available.
Another reason not to do online check-in is if you're buying park tickets and want them added to your room key...
But if you're just taking whatever room you're assigned, and putting your credit card on the account, and have no other "business" w/the check-in folks, then online check-in will save you maybe 5 minutes...
Personally, I don't find it a big deal. But I usually do it, if I have no reason not to, just because it feels so good 10 days before I leave to "check in"...kind of accelerates the magic just a bit!