Update 1.10: 2 New InstaMats:
Update 1.10 added 2 new InstaMats: Eevee_Glass and Emission. A new popup node editor and 2.91 support were also added

What is InstaMat?

InstaMat is a pie-menu addon (Shift + F) with 3 main goals: Making it faster, easier and more versatile of:

1) Editing materials you have in your scene

2) Managing materials you either have in your scene, or use often

3) Creating so called 'Standard Materials' in the form of 'InstaMats' like metal, fabric, wood, glass and plastic. Versatility, detail and performance oriented materials, that can be created with 1 click, that rival the looks of your best PBR materials.

Known limitations:
I think you should know the limitations of this addon before you decide to buy
1) Adding an InstaMat for the first time is instant in solid mode, but may take some seconds to load the textures in LookDev/Material mode. (Note that this is the same as with any texture based material, and also all InstaMat's of the same type applied afterwards load basically instantly (Since then the textures are loaded, and they all use the same ones))
2) When opening the pie menu too quickly, it may close again automatically. It's a matter of waiting a quarter of a second before moving your mouse again. This is a limitation of implementing larger menus inside a pie menu in Blender

Editing features:

Managing features:

Creating: InstaMats:

You create an InstaMat by pressing one of the InstaMat buttons in the pie menu. After clicking, a new popup will open that allows you to customize the material. (The first time you add for example a Metal InstaMat in your scene, it takes a few seconds to load the textures. Afterwards adding new Metal InstaMats is basically instant.)

Example of a Metal InstaMat, using the preset 'Aluminium'. Note that presets are just a saved version of the settings shown in the panel. You can create anything you can imagine with the sliders shown and then save it to your own preset

InstaMats can be changed any time after creation. The QuickAdjust panel in the pie menu shows the exact same settings as you see when first creating the InstaMat. Note: the material you see here is the exact same material as in the picture above, just different slider positions.

Credits:

Shader Ball model used in pictures is CC BY 3.0, Created by Robin Marin