We weren't able to get a reservation in advance, but on our Epcot day, I walked over around 10:15 ( maybe a few minutes before) and got in line. There were maybe 3-4 groups in front of me. They come out and offer the option to stay in line for the lounge (bar seating) with first seating around 11:30 and you have to stay in line... but she said that was probably the shortest wait of the day... or when it was your turn, you could see if she had a reservation available. I preferred lunch, so she had like 3-4 times for me to choose from for a party of 4 and we were able to have lunch. There would have been dinner options as well, so it sounds like they have quite a few spots available day of.
Unfortunately we did orange mission space (without dramamine) before lunch, so I didn't enjoy the experience as much as I would have liked. The restaurant has a weird odor, I think from the smoked dish... so it made me more queasy. I ended up reheating most of my meal later at the resort (it was still pretty good!) the waiter was great though, and brought me ginger ale and tea when I got hit with another wave of yuck. (I was feeling okay but the bacon on my burger was way too BBQ scented for me and smell on the first bite brought back the icky feelings)