I had two computers running this morning and was able to get one site. And I got lucky! You need to find the smallest spot you can fit into (the bigger ones are in more demand) and then another one in case the first one gets booked. I signed in at 6:30, picked out two campsites, and kept pushing the "book this site" button to keep the selections active. You need to push the "book this site" at just the right moment, and that's pretty hard to do.
I'm really excited about being able to go to Yosemite again, we didn't get to camp much last year.