There's really no downside to using online check-in. Its only purpose is to get your paperwork and key prepared before you arrive.
Now, after reading bunches of reports on the Resorts board, and from my own experience, I can say it's not so much a case of 'is it good or bad', as 'there are appropriate and inappropriate situations for online check-in'.
When I stayed at AKV in March, I didn't particularly care where my room was, it was just me staying, I had no questions for the front desk CMs, I didn't have to add anything or change anything. I walked up to the counter, gave them my name and showed a photo ID, they handed me my key and packet, and I was off to my room. The whole transaction took a minute or two. I was sort of the ideal guest for online check-in.
I've seen lots of reports, though, of people doing online check-in who want to do things like upgrade their room, ask for a different room location, have all kinds of questions for the front desk CM, or do some sort of complicated transaction with their tickets. Which eliminates the one benefit of checking in online - saving time. And, if you get a couple of those people at the desk, the previously fast-moving online check-in line becomes just as slow as the regular line, and I think the CMs get frustrated because the process gets bogged down.
So, I would say, if you have a straightforward situation where you can pick up your stuff and go, online check-in is great, use it. If you think you may need to make changes in your room assignment, or have any other complicating factor, go with the regular check-in, because online check-in won't save you any time.