1) What brand do you like? - I have only used ebags Brand and LOVE love love them!
2) What size(s) do you recommend? - I bought the small, medium, large set, the slim or narrow set, the pack-it-flat toiletry bag, and the padded pouches. I also just bought another small bag because I needed a filler item for an eBay coupon code. You can never have too many!
3) How do you pack them? (outfits separated into individual cubes, shirts in one, socks in one, etc.??) - There are a ton of YouTube videos on packing so that might be a good start, but it really depends on the trip. I used them for a slit stay at WDW last year (2 nights in each hotel), and had clothes for AoA in one and clothes for when we moved to PC in another. That way, I could easily "unpack" for each hotel and pack up my dirty clothes and put them back in a cube when we left. For any other long trip, I can usually fit most of my clothes in a large cube. I use the small one as an organizer for my carry-on stuff on an airplane. It fits the iPad perfectly, a book, some snacks, etc. I just throw the cube in my carry-on bag and can easily access what I need without having to shuffle around a bag to get it. I use the padded pouches to hold all of my cords, chargers, memory cards, etc.
4) How many do you think one adult needs? One child? - I have all of the items listed in #2 but I have never used them all at once. Since I have started using the ebags cubes, I also bought the Jr. Motherload Backpack as my primary piece of luggage, so not nearly everything would fit in there. I probably use the large or medium size the most, along with one or two of the slim cubes. I bought my child a set of the small, medium, and large in his own color. The small is great for an airplane worth of activities, and the large is big enough for clothes.
You really can't go wrong. I would love to purchase more and more, but I really don't need anything else. I might get some shoe bags though sometime in the future.