I think it depends a lot on how much cutting you do, which shapes are your favorite cuts, and how good you are with a rotary cutter.
The thing I like most about my Go! are the triangles. They are cut perfectly with all the points nipped off, so they are easy to match up to sew accurately. The tumbler shape would be difficult to rotary cut, so I like that die, but tumblers have limited design possibilities.
For strips and squares, I can do just as well with my rotary cutter and a Shape Cut mat.
The applique shapes are primarily for use with fusible applique.
The Go! does make it easier for me to cut a lot of shapes in a day, so it is just borderline worth it for me. For those with problems using a rotary cutter, it can be very worthwhile.