I have a bunch of the eBags cubes (too many actually - you don't need as many as you think you do!). I use the medium and small the most. Sometimes I will use 1 large one, but usually just on longer trips. I took 2 medium, 3 small and 1 large for my clothes on a 12 day trip overseas this past summer. They accommodated 1 summer-type outfit a day (typically a skort and top for me), plus a few extras for good measure, along with swimsuits, undergarments, pajamas, a cardigan and even a rain jacket. The key is to roll your clothes and really stuff as much as you can into the cubes so they compress things some. I tend to stuff tops/bottoms of outfits together into the same cube so I don't have to dig to find things, but that's really up to you. Sometimes I put all tops in one, bottoms in another, etc. Just depends on the nature of the trip and what I think the organizational issues will be.
I like the eBags cubes - no quality issues, zippers seem strong and all that jazz. They seem to be some of the best deals for good quality bags in terms of price, that's why I went with them.
I also have several Eagle Creek Pack It Half Tubes for things like first aid supplies, electronic and other small bits and pieces, etc. I like these because the extendable handle on my suitcase is inside (as opposed to on the outside of the bag) and these fill in the spaces between those poles perfectly.
ETA: To give another perspective, for a 4 day/3 night trip recently, I used just 1 medium and 1 small cube and they weren't completely stuffed. I don't put makeup and toiletries into the cubes, I use a makeup bag for those. And shoes have their own shoe bags.
