I never really thought that traps would be very helpful, especially while farming. I always overlooked them and didn't think they were worth investing in. Even to this day I am still skeptical about the cost/effectiveness of giant bombs, so I have decided to look into it and do some testing within my base. Now I shall present my results to the clash community. Keep in mind that this is my personal experience/luck with the traps, and if you have better ideas please comment.
The classic bomb. Everyone has bought one before expecting a "BOOOOOM!" and receiving a tiny little explosion dealing 12 damage over a radius of 3 tiles. Bombs are practically useless, unless you group 3-6 of them together. When grouped together they can effectively take out a group of goblins or archers running by. Otherwise bombs will just slightly weaken enemy troops making the clean up for your defenses slightly easier, which could be the difference in an intense battle. Despite being by far the worst trap in the game, bombs, if used correctly, can be helpful.
#2 Spring Traps
Everyone loves spring traps, myself included. These used to be the only traps that I thought were effective enough for their cost. Spring traps cost 2,000 gold where the bombs only cost 400, so you expect more from them, right? Other than their poor activation radius and the fact that goblins can run right over them spring traps are far better than bombs. Bombs damage existing units and leave them for your defenses to finish off while spring traps literally throw units off the map. This leaves nothing for your defenses to shoot at letting them focus on the other advancing units, potentially saving your base. Unlike bombs spring traps are better spread out around or in your base. If your farming they are good to put near your town hall in defense. Spring traps are almost impossible to use incorrectly and will usually get the job done, therefor I would say that spring traps are cost effective anywhere.
#3 Giant Bombs
Giant bombs crush units with a massive force of 110 damage, but they come with a price that will hurt your wallet. 50,000 for a one time only explosion. Really?
I have never been a huge fan of giant bombs because I am farming, so they hurt my gold levels a lot. Not only are they expensive but they are commonly misused. I'll start to attack someone with a few archers on the side when "BOOOM!" a giant bomb goes off killing all 2 of the archers there. I start laughing and thinking in my mind...WHAT A DUMB ___ (you know the word). NEVER EVER put giant bombs on the outside of your base unless you would like to waste your 50k gold. Giant bombs need to have a little place to chill inside your base in a heavily traveled attacking path. They are great for covering weak spots INSIDE your base. Even inside occasionally they will go off on a wall breaker or 5 goblins and you will question the profitability of the bomb. So you just have to know whether or not its worth it to spend 50k to protect the 200k that attackers can take. The best way to find out if it is worth it on your base is to try it when you don't have a lot of money. Giant bombs aren't very cost effective in my opinion, but I think that this is one of the most controversial traps in the game.
#4 Air Bombs
Air bombs are basically the flying version of bombs. They deal 100 damage in a radius of 5 tiles and cost 4,000. I like the air mines, they shred up balloons like wizard towers do and they will hit clumps of minions/healers/dragons as well. They will only be devastating to balloons/minions, but they can certainly slow down other units, leaving them weakened for your air defenses. I like to put them in clumps of 2 outside my walls or just inside them. They are decent traps, however I think that they are over priced for what they do. If they were 3,000 instead of 4,000 then I think that they would be totally worth it. In conclusion they will preform well and can be very useful against clumped flying units.
#5 Seeking Air Mine
This is the granddaddy of traps. This air mine will take out almost any flying unit in one shot. It deals a massive 1,500 damage and is only incapable of taking out dragons in one hit. The seeking mine does not do splash damage and will not target minions even if they are the only flying unit in range. It costs 600 dark elixir so I don't recommend buying them constantly until after you buy your barb king. There is not much else to say other than DO NOT put these right on top of your air defense, they are expensive and you want to maximize their ability to destroy air units. They are very cost effective as well, they can basically stop a dragon dead in its tracks.
Concluding this guide I would like to thank those who read it and feel free to comment. I would also like to thank http://clashofclans.wikia.com/wiki/Traps for actual bomb radius/damage data. Hope it was helpful, check out my bases and feel free to ask me questions and most importantly check out my clan APOCALYPSE3