Very useful and easy to use.
I purchased the Amp Gizmo in order to simplify the patch- and channel-switching in my guitar rig. I currently use a Boss GT-10 through a Mesa Boogie Roadster, and I was able to integrate and program the Amp Gizmo into my rig in a very short time. The Amp Gizmo allows me to program channel selection at the same time I change patches on the GT-10. It uses a MIDI signal from the GT-10 to signal the Amp Gizmo to change any parameter formerly controlled by the footswitch. It completely eliminates the Mesa footswitch, and combines what was two button choices into one.
I lose the flexibility of being able to choose the amp channel for a given patch, but that's more than made up for in being able to program entire song pallets, along with channel selection, across a bank on the GT-10. This is important, since I'm the only guitar in a bar band and need to switch patches and channels seamlessly.
The only down side is the cost of the proprietary cable from the Amp Gizmo to the amplifier of choice; however the alternative is a very messy pile of patch cables. They could also have thrown in a MIDI cable, which is also necessary to make the thing work.