I used 1/2" sandply (maybe it was sanded ply) from Home Depot. Tim said that was what he used. It's very nice and solid plywood and it's smooth too. About the same price as their birch plywood only it looked better, IMO (more plys as I recall). I had Home Depot cut it into sizes I could manage and they did that at no extra charge. Also helped with getting it in my old station wagon, er, SUV as they say these days. I have some pictures from when I assembled mine, I'll see if I can find and post them, they are here somewhere. Layout is not finished as I intended to add on to it, but after 3-4 years now I still can't decide on a final plan...
Found the pictures.
This was their 6' x 16' kit. It took me about 8 or 9 hours total, including unpacking, reading the directions, sorting all the parts, going to get the plywood and installing it. Home Depot is fairly close, within 8 miles or so and I went on a weekday so they were not busy. One thing I would probably do differently is make the plywood extend out past the legs and frame. I may still do that when I add on or at least add a piece where no adding on is done. I think it looks good as it is, but I have seen others post theirs with the extra sized top pieces and it looks very nice. Nicer than without, IMO.
Note: The Mianne benchwork is made center of leg to center of leg (or center of frame to frame). I think they explain that on their website or in the catalog? But the actual kit is I think 1-1/2" or so larger all around (5/8" or so per side). If you are going wall to wall this matters and needs to be taken into account. If you are putting it in the middle of the room it only matters if you want the extra size to cover the legs and frame. Hope I didn't make that too confusing?