I was always anti-packing cubes because I couldn't wrap my head around how adding something else to your suitcase allows you to fit more. Out of sheer curiosity, I ordered a set of packing cubes from eBags - it was a variety set that came with three oblog cubes, and then small, medium and large ones. I used these on our last trip to WDW - I'm now a believer!
We found the medium one to be the perfect size. The large one was good but took up most of the surface space of our largest suitcase. With two medium ones, we could put them side by side in the suitcase. The small and oblong ones were good for our socks and underwear. While I still overpacked and ran out of room in the luggage (and pushed the weight limit), what I actually liked most about them was the organization. When we got to the resort, I just pulled each cube out and put them in the drawer. I knew which cube held what, and obviously unpacking/packing back up was much easier.