Another bonus to knifing on the tac switch is you can EASILY wear out a controller by using the sticks to run or knife. Those sticks take a ton of abuse from your thumbs so by re-routing the stick presses to the tac switches you really help the life of the controller. I can honestly say that it tends to be your dominant thumb that kills the controller the fastest. I have broken PS2, PS3, AND 360 controllers ALL in the same exact way... R3/RS drifts on its own and loses accuracy.
With this I can just map the turbo knife to the bottom right tac switch and easily tap it in a panic. It knifes 3 times for every one normal knife and you wouldn't believe how many times I have gotten jumpy and accidentally hit the button to knife and actually killed the guy.
These controllers ARE expensive, complex, and frustrating when you are trying to get it just right. It will take you days or even weeks to figure out what you honestly want. The difference is you can have a true jack of all trades controller as opposed to a one trick pony that kills over a few months down the line. You can set it up for whatever game you want, you can have pink LEDs, you can make a button to teabag anyone you want, and you will eventually get in the habit of using rapid fire on a toggle in a way that allows you to easily keep your accuracy at range. No annoying hitting the sync button or any of that crap to turn your rapid fire off.
General rule of thumb is if you want accuracy at range with a semi, turn rapid fire off, if you want it at range with an auto, turn rapid fire on. It is really that simple.
No it won't increase your KDR, make you some god, or make you magically invincible. You will have to learn to set it up to your play style then most likely adjust to it but yes it WILL make your life a little easier. CoD is a tricky game... the main factor in it is your connection. The controller won't save you when your connection sucks and you may even have issues with rapid fire at that point. I do a LOT on DSL. I had to slow mine a little to get it to be more reliable. Just play around with it and you will see.
It is by far the best modded controller on the market though. My friend has a competitor and his can't do anything at all better than mine and I can easily switch pages while he is fidgeting with god knows how many button presses. If you are going to spend the money on a controller just spend more on a Viking so in the long run you don't waste even MORE on other controllers once yours gets patched or a new game comes out.