I bought this grinder when my Rancilio Rocky died, so I can compare the two. The most noteable difference is that the Mazzer is built like a truck. It feels like a serious grinder and is a great complement to a machine like the Livia 90. The grinding is consistent when maintained at the same setting, and small adjustments can be made easily. Unlike the Rocky, which sometimes gets stuck and needs some good whacks to the side to get going again (with oily beans), the Mazzer always grinds perfectly.
The machine is large (despite the "Mini" name) and quite expensive...nearly twice the price of the Rocky. However, I think the solid construction, more consistent grinding, and lack of jamming are worth the difference if you are buying a new grinder and don‘t already have a Rocky. I don‘t think it‘s really necessary to upgrade if you already have the Rocky unless you can get a good price for it.
This is a serious machine and will last you a long time.