Very useful boxes with heat ridges and easy means of sliding in PCBs. There is a good range of sizes, and because you can by the back plates separately you can both make errors, and buy another, or try something out and do it again properly! The existing T slots make aligning fittings easy, they are perfect for prototypes and small runs.
I would like some rubber feet that slid into the extrusion T slots, and some joining plates so you could use combinations to extend a size as needed. I think some pre-cut plates for XLR, switches etc. with plastic clip in infills would make the whole range very versatile.