Yes, it's worthwhile doing. You can't enter any 'free-form' text requests into the form, just a couple of basic tickboxes, but your requests should ideally have been added in advance to your main booking so that won't really make any difference. After you have done online checkin, you are still free to send a fax 3-5 days out if you wish - if anything having done online checkin might help them, as they'll also be able to see your arrival details, etc.
The main benefits are: you can register your credit card and create a PIN number for all your key-to-the-world cards in advance; nothing to take up time at the front desk (your welcome pack, key-cards and room number will be put together a few hours before your stated arrival time, so it's all there waiting for you even if you are checking in quite late); when MagicBands are introduced, they will have been pre-made for you in your chosen colours and with your names on (otherwise you'll get generic grey ones set up at the front desk); you might even get 'Mobile Lobby' checkin service, where they can check you in using an iPad, rather than even needing to queue for the desk at all. The room assignment team suggest it too by the way, they say it generally speeds the whole process up for you.
The only downside I've heard anyone say is what happens if your requests have not been met and you want to change from the room you have been assigned? Well if that happens you're only in exactly the same situation as if you'd queued up for normal checkin anyway, you can still ask if there are any other options available and they can move you if one is found. And with the new RFID keycards they probably won't even need to print you new cards, they'll just re-assign the ones you've already been given.
So all-in-all, I can't really see any good reasons why not to do it. There are lots of pluses, and no real negatives that I can see.
