Skinning layers are a central feature of ngSkinTools. With them, you break your rig down into easier manageable parts and edit them separately, then blend everything together through layer transparency.
They're not just a simple way to make your work more organized - they also physically isolate groups of influences from the rest of the rig, so paint and edit operations won't mix-in influences you were not expecting. This also allows you do things that were impossible before: per-layer mirroring, adjusting influence weight up/down through layer transparency, blend transfered weights with previous weights, to name a few.
You won't be needing to go to T-pose again when going through paint+mirror cycle, you can mirror in any pose now. There's also a much greater control over what influences mirror onto which one: associating left-right influences by name without prefix, manual association overrides, etc.
Mirroring is also layer-friendly, so you can mirror just the parts of your model you're currently working on - very helpful, when you want to keep other parts of your rig intact.
ngSkinTools has it's own set of paint tools, which are capable of working on a per-layer basis. Each tool maintains it's own intensity, which is handy when toggling between smooth and replace through shift-modifier. Smooth is completely redesigned and you're going to just love it:)
The new smooth brush gives much better results, as it operates on all influences of a vertex at once. For even greater control, the more precise "relax weights" tool is there, which also can smooth across mesh boundaries and thin meshes.
ngSkinTools operate on standard Maya's skinCluster (also known as "smooth skin"), so no custom nodes will be required to use your rig. Plugin has a couple of custom nodes, but they're only required until the rig is complete, and can be deleted after, so your work should stay compatible with most pipelines out there.
ngSkinTools is primarily a C++ plugin, with only it's UI implemented in Python.
This means that compute-intensive operations are being handled in the most efficient way. Of course, with layers and workarounds needed to add something nonstandard as this into Maya, it's still somewhat slower than standard smooth skin paint.