I like being able to enter my cc info myself, instead of handing my card over to the CM at the desk. The fewer people who handle my card, the better.
The advice I would give to someone who hasn't tried it yet is this - if you have a room request that will make or break your vacation then do NOT do online check in. Although your room can be changed, it takes a lot of time, which negates any time savings you would net by using online check-in in the first place. It's better to go over your request with the CM so you can find the room in available inventory that best matches your request.
If you just have a basic request, or no requests at all, then online check in does save time, and since they know when you are arriving, they do try to have a room available for you when you arrive (not guaranteed, of course, but so far I have had one every time, even checking in as early as 10am). I don't make a lot of room requests, and so far the rooms I've been assigned have been in very convenient locations so I've been happy.
With online check in moving to 60 days out, there's no way you could select your specific room. There are too many variables with that long a time period. I stay at a lot of Hilton properties, and they do allow you to select your room during online check in (at least, as a Gold member or higher) - but they don't allow you to do the check in until 36 hours out from arrival. I actually prefer being able to select my room from the inventory they show on the screen and I wish Disney did that too even if it meant that I couldn't do online check in until 36 hours out.