Thanks for the plans. My little brother from Big Brothers Big Sisters and I made this table and had lots of fun doing so. We made a few modifications which might be useful to others:
1. We modified the center joints (at the lower and table level) so that one 2x4 could continue across the entire diameter of the table for greater strength (see pictures for details).
2. As recommended we extended the seats out 4". This made it very comfortable to get in and out.
3. We started the center with 2 pieces of 2x6 rather than 6 with the thought being that this would help strengthen the center of the table. (see pictures for details).
4. We added a stiffener at the center 6-way joint on the lower level to help really strengthen that center joint. This was made out of 2 pieces of 2x6 as shown in the pictures.