If you like to do rag quilts, the rag dies are really nice because it cuts the raggy part so no more having to take snips to make it raggy looking. But I've heard it really chews up the cutting mats so buy extra mats.
I think it may be better to buy the odd shaped dies, pieces that are a little harder to cut by hand.
Straight squares are easy enough to do by hand and most of us do strip piecing then cut strips into the size we want.
I do like the 6 1/2" square for my scraps.
The triangle die has the corners already trimmed off which is very nice.