It really depends on your budget, how much cutting you need to do, and what shapes you are interested in. I have an Accuquilt Go! and like it, but it gets pricey as you add on more and more dies.
If mostly you want to cut strips and squares, then I would advise getting a June Tailor Shape Cut mat -- much less expensive, and just as fast.
The Go! is really nice for people who simply can't seem to attain accuracy with rotary cutting. It is also very nice for cutting triangles (they piece much more accurately) and odd shapes such as the tumbler.
It is definitely not a necessity, so you have to decide if it's worth it within your budget constraints. For a lot of people, it won't be.