I hadn't heard of them before, Graham, until I saw mention of them on the Forum. I agree that they are very useful, especially when touring around.
I bought mine from Amazon and went for a pack of small ones (fine for keeping things like chargers, adapters, socks etc); then the next size up, 3 or 4 of which will fit across my suitcase and take undies, t-shirts etc; and finally, the next size up from there, 2 of which will fit side by side on the top and take sweaters. I have to admit that I rarely use the biggest ones, preferring to put the bulkiest stuff in the lid of my suitcase, but the others are always useful. I pack them neatly when going on holiday, keep them on the wardrobe shelves of my room (or in the suitcase if moving on each night) and use one of them to hold the dirty laundry on the way home. I haven't yet done it but it would be possible to use one to keep all the stuff together for the following day to save having to unpack the whole suitcase if touring and arriving somewhere fairly late in the evening.
My suitcase is black so I bought turquoise cubes, that way I can see them easily in the gloom of the interior. As they come in several colours, too, you can just go for whichever makes you feel most cheerful when tackling the packing/unpacking/packing.
So yes, I would recommend packing cubes - if you get them, enjoy the holiday that goes with them.
Ann