One of the interesting things about lifehacking is that it seems to be one of those things where "a little knowledge is a dangerous thing" - kind of like the kid who takes Philosophy 101, learns a little bit about syllogisms and formal deductive fallacies like ad hominems, and goes around being obnoxious while never realizing that actually, most deductive fallacies are valid inductive arguments. (If a Nobel physicist claims something about a random point in quantum mechanics, he really is more likely to be correct than a guy sitting next to you on the bus, even though if you wrote this down on a syllogism test, this would be flagged as an 'argument from authority'.) We might call this "the valley of bad rationality" http://wiki.lesswrong.com/wiki/Valley_of_bad_rationality

In the case of lifehacking, it's more a case of "the valley of bad economics": of knowing just enough economics to understand that saving time is valuable and worth paying for upfront, but not being good enough to take the analysis any further and consider things like 1) how much time you are actually saving on net, 2) how long you are likely to be using any system and receiving the gains you paid for, 3) and whether you might not be overestimating both figures.

Frequently, you find that even large apparent gains are not actually justified solely on the basis of saving time because of the uncertainty and discounting of future gains you need to do.

To give some examples I recently worked through some numbers on various self-experiments I've done: http://lesswrong.com/lw/cih/value_of_information_8_examples/

The striking thing is that even the most dramatic self-experiments like using melatonin to improve my sleep did not justify very time-consuming (and more reliable) experiments, and the most efficient thing to do would just take melatonin on the strength of my background information and subjective impressions of efficacy.