Being organized is hands down the most important thing when trying to pack light and fast on a RTW trip. After researching packing cubes, I have decided to go with the Eagle Creek Specter Compression Cubes. The pair cost me $38 which contains one that can be utilized for shirts and another for underwear. I will probably get another set before my trip which I will use for shorts/pants and socks.
The fact that I can compress 6 shirts easily into a pancake, gives me the benefits of being organized and not taking up much room. Basically you pack up the cube, then zip the side compression zipper which compressed the cube into a pancake.
Eagle Creek Specter Compression Cubes