They do give a small amount of horse power increase and maybe some increase in mpg if they are designed SPECIFICALLY for your vehicle. They usually look pretty cool. You will probably hear your intake sound now when you accelerate. Some installs are easy, some are hard. For every day street use you're probably just throwing money away for the amount you gain. However, you said you need to replace your air ducts so that would be money off the price of a kit. In other words, how much more would it cost or is it even cheaper to get a kit. I think I would stick with the kit from K&N if available.
Make sure they are not eliminating the "hot" air intake if your vehicle has one. This would be an intake hose that runs to the exhaust manifold to get hot air to assist in engine warm ups and cold weather driving.
