I have some, and I use them -- or don't use them -- depending upon my needs:
They're great if you're packing two or more people's gear in one suitcase. Upon arrival, you toss his blue cubes into his drawers, your red cubes into your drawers -- done. No mix-ups.
They're great if you're staying in multiple hotels on your trip. Quick in, quick out.
They're great if your suitcase is searched -- no stranger has to touch your undies.
The small ones are great for packing a kid's complete outfit -- here's everything you need, go change. This is particularly nice for campgrounds, where the bath house is a walk.
Whether they keep your things "nicer" and less wrinkled depends entirely upon how you pack.
However, you will get more into your suitcase if you just pack things in. If you're packing everything together, you can put in your pants/shirts, then shove your socks and swimsuits around the edges. In a cube, however, your underwear-cube takes up just as much space as your shirt-cube. The cubes themselves also will add some small quantity of material and weight -- this would matter for a backpacking trip.