Demi-glaces --- is anyone familiar with these brands?
No, they're not worth getting, in all sincerity. They are crap and will make your food taste like crap. It's like cooking with a cheap, commercial, over-processed wine. If you can tell the difference between good wine and bad, you will not be happy with commercially-prepared, shelf stable demi-glace.
Making your own is not that hard, and you make a lot at once and freeze it to use incrementally. It sounds like more effort than it it - not effortLESS, but cheap and mostly simmering without your direct attention.