This planter box turned-out looking great! I put it together the first go-round using all kreg-jig screws, but couldn't get the pieces to sit flush with each other, so I took everything apart...and used star-drive screws instead of nails). I have decided that I hate nails.
I used the kreg jig to attach the 4 posts, but then used brackets to attach the rest of the panels -- this worked well! Once I put the slats in the bottom, it was very sturdy.
TIPS: don't be afraid to alter the plan. If you like working with brackets, use them!