I bought dies that are sized to all fit together, specifically for using up my scraps. I bought blocks that finish at 3" and 6", and rectangles that finish at 1.5x3 and 3x6. I also have the blocks that finish at 4"....so between all of those I can build all sorts of variations of 12" blocks for a nice scrappy quilt, and also hopefully cut the maximum number of pieces from odd-shaped scraps.
I also like the rag dies, I bought the circle one and will probably get the square rag die eventually too. I will NEVER make another rag quilt without using the die cutter!
I haven't bought any strip dies yet - I have the big June Tailor mat that cuts 2-1/2" strips and so far that suits all my strip-making needs.