One of the fundamental causes of low software development and enhancement projects success rate are improperly derived estimates for their costs and time. In the case of such projects the budget and time frame are determinated by the effort being spent on activities needed to deliver product, which would meet client's requirements. Objective and reliable effort estimation still appears to be a great challenge to the software engineering. In the author's opinion the main reason of that problem is effort estimation on the basis of resources, while such planning activity should ground on the required software product size, which determines work effort. But using suitable methods for software size estimation will not be sufficient as long as it does not take into account benchmarking data concerning similar projects having been completed in the past. This paper is aimed at indicating the important role of using reliable benchmarking data in order to obtain valuable effort estimates for software development and enhancement projects, including particularly the attempt to analyse usefulness of benchmarking data of general character.