If you are making an improvement that takes away an eyesore or a negative that a buyer would strongly object too if there is not an improvement, the value in the buyer's eyes will be higher. It's not that you will get a higher offer as a result of your improvements, but that you will not be "dinged" because of the negative that existed before the improvement.
If however, you are getting a personal benefit - you aren't exactly throwing your money away. A well landscaped house - when compared to the same product without good landscaping - will probably sell much quicker. And as they say - time is money.