These cubes revolutionized the way I travel. If you're on the fence about packing cubes in general, make the plunge and fit, like, 50% more stuff in your bag. Plus, you'll know where all of your stuff is ALWAYS. My friends and fellow travelers are always jealous of my packing cubes. You'll never lose anything in your bag again. Say goodbye to the days of unpacking your entire suitcase for that one hard-to-reach item!
I am always on the lookout for a better one than this eagle creek clean/dirty cube, but have yet to find one. The dirty compartment makes this cube my favorite. It's SO convenient to separate wet, smelly, or soiled clothes from the clean ones. The dirty side keeps moisture and smells in very effectively.

This size cube can fit 4 outfits or 2 pairs of shoes comfortably. Maybe up to three beach towels. I could cram a week's worth of clothes in one of these (which comfortably fills half of my 30-L backpack)