So we've used them twice now, me and DS2 (DH and DS1 not interested unless I do it for them!)
Pros
They make it massively easier to unpack and pack when away as you just take the cubes out and put in drawers
If moving hotels they are great as the stuff is all in cubes and stays in cubes.
They make it tidier when away.
It's easier to see whose clothes are whose (different colours)
Cons
I've needed more smaller cubes - DS2 needs more bigger ones. (I tend to take clothes on hangers, so need more small ones for underwear and small stuff, he has jeans, jumpers, t-shirts etc.) The ones I have are Medium/Small assorted. Because I have different colours I can't swap them around!
You need to think about how to use them. I've seen people use lots of smaller ones with an outfit per day (great for kids), but if you are an adult or older child reusing the same clothes to reduce clothes taken you need bigger cubes for types of clothes.