I'm not sure what more I can say; they're nice & well-made, yes.
Having said that, I think it's a bit pointless to consider pyjamas as something to "invest in". They're the kind of item that gets washed & tumble-dried so often that regardless of quality, they're going to wear out sooner than items you naturally will take a bit more care of. Spend as much as you want, without expectation that they're an item that will last forever. Linen PJs would probably outlast cotton, FWIW. If you want to really spend money on luxury jim-jams, there are more expensive pairs around, and finer materials like silk. Other brands I've occasionally heard mentioned on here (but that I've never tried) are Zimmerli and T&A. I think someone here had Hermes PJs too, IIRC? But for me, Derek Rose are enough of a step up over the more typical M&S pairs, without being really silly money. After all, you're unconscious for most of the time you're wearing them, so I don't think it's worth spending vast amounts on them! :)