It's nothing they don't already have 10-trillion times anyway - just the usual stuff - name, date entered Thailand, birthday, address - nothing more personal than that. It's actually much less information than you have to submit when doing the report personally at immigration.
When you do it online, they send you an Email when the next one is due (which they don't do if you submitted at immigration), an Email letting you know your submission was accepted, and one more Email letting you know if your application was approved or rejected.
My unsolicited advice is to try it once. If you like it, now you have the option of doing it online or going to immigration. You can do it up to 14 days in advance of your current due date. I always do it well in advance in case mine gets rejected. That still gives me the better part of 2 weeks to get to immigration before the current report expires.
