We have a vertical display garden where I volunteer. The gutter and bag gardens have issues with low soil volume so you cannot plant things that are large in them and the shallow volumes and the fact that they are off the ground exposed to the wind means they dry out very fast. Ours has a drip irrigation system to water it but it needs to be watered a couple of times a day and if the system fails for any reason, the plants don't last long. The soil eventually gets lost especially in the hanging pockets.
Vertcal towers can work quite well but usually using all of the pockets is sometimes impractical depending on what you are planting. If you are building a tower just for strawberries it should be fine but if you did a mix of plants some plants could be come invasive like mint and take over or large plants like tomatoes will have large root systems that use up a lot of the nutrients.
Then there is the problem of space. If you have large plants hanging out they run into the other plants and usually it is better not to plant all of the holes in that case.
Tower gardens have to be watered from above so it is best to install drip tubing and attach it to a timer or faucet to make it easier to water. The tower needs to be unobstructed on all sides to plant all of the pockets so it is usually out in the sun. If you have one side up against a wall, you cannot plant that side. The barrel is good because it has a lot of soil volume and can be made with its own compost bin. You need the bottom cover to clean out the compost bin though and it has to be raised on blocks because it would be heavy to move once it is filled. I would offset the pockets and put in fewer pockets to give each plant a little more growing room. Plants on the top will need to be able to handle drier conditions and plants on the bottom will need to be able to handle wetter conditions. I made a tower out of different sizes of pots a 20 inch pot ,14 inch pot and an 10 inch pot. I drilled a 1 inch hole through the center to put the closet rod that I used as a stake to keep it from falling over. It gave me about 4 inches of planting space on each level. I had to choose plants that trailed on the bottom so they would not interfere with the middle level which had plants that grew upright. The top I had a basil plant. It was relatively easy to water it and take it apart since the pot could be taken off the pole. Eventually the soil had to be replaced. It also had to have light all around it for it to be planting 360 degrees. The other thing that worked up against a wall was to create a stadium by using tiles I created a staircase where I could put potted plants. By having different levels It gave the plants more space especially for the canopy to spread out on each tier. Since this not a tiered planter, which is another idea you can use, I could move the pots around to make the best use of the space so the growing plants would not interfere with each other.
http://www.amazon.com/Greenes-Tiered-Ce ... B00C3MJMO8
Happy gardening in Hawaii. Gardens are where people grow.