I'm not a zen master or yogi, but I can promise you that if you try to measure the results of meditation (Am I feeling better yet? How about now?) you'll wind up chasing your own tail and giving up. Anyone promising wonderful results through meditation is very likely selling you something.
The effects of meditation are best compared to those of sleep. A good night's sleep doesn't make up for lousy sleeping habits the rest of the week. But getting enough sleep _regularly_ is key to being healthy, happy, and productive.
Ditto meditation. Trying it once in a while is likely as waste of time. But practicing it regularly (and getting better at it, which you will, with practice) can have positive effects, over time.
As an aside - Zen takes a good deal of commitment, but it's the least 'bullshity' of all the approaches to meditation/mindfulness I've encountered. Eight Gates of Zen is a great intro: http://www.mro.org/zmm/training/eightgates.php
This attitude of "you can't measure it, don't even try" really bothers me. You can measure the benefits of both meditation and sleep, and there have been studies that do so. This wikipedia article is pretty well-cited, and outlines many of the studies that have been done on meditation: http://en.wikipedia.org/wiki/Research_on_meditation
It's not that you can't; indeed you can. But actively looking for results means the sort of outcome dependence which you are trying to avoid through meditation.
In the Eastern religions this is very much what is referred to as "faith"; the study and analysis of meditation in this manner is intellectual, but meditation itself if very much anti-intellectual and seeks for the non attachment of intellectual concepts (a difficult concept to grasp for those of us raised in the rationalist philosophies of the West).
It doesn't mean that you can't measure it; it means that, aside from the sensations which come to you of well-being and effects that come to you from direct observation, you shouldn't be using it as your primary guide to the effectivity of meditation. Fortunately there's 3000 years of incredibly descriptive and academic Buddhist and Hinduist texts which talk about this in amazing depth, and have zero usefulness for your personal progress.
It bothers you? Awesome! That's a rejected shadow you can investigate through mindfulness investigation. I guarantee you, you will get results and realize insights if you get in touch with that feeling of being upset when someone says "you can't measure it, don't even try." :-D