^ I'm not sure if all of those little TMNT / Usagi crossover shorts that are in the two volume slipcover are included in the individual volumes. Otherwise, it's the same material originally published in volumes 1 through 7.
Of course, the Usagi / TMNT collection Darkhorse just put out has everything they've ever appeared in together, so if you need to fill that hole in your collection it's pretty easy to do so.