There's a claim that evolutionary explanations are nothin more then "just so stories", that is, for any trait we can find in nature, one could find a way to explain why it's adaptive.
EDIT: That's actually two false claims!
Claim 1: For every trait we can find in nature, one could find a way to explain why it's adaptive
This claim is false for two reasons (1) Not every trait is adaptive, and (2) At the moment, it is not technically possible to test many of the explanations.
Not every trait is adaptive: Evolution, as in the change in the genetic make-up of a population, is more than "survival of the fittest". The genetic make-up of a population changes due to mutations, genetic drift, recombination, and selection. Only the latter is adaptive. The three former are non-adaptive, but can still give rise to traits that we can observe now. See for example this article for a discussion of why it can be wrong to always look for an adaptive explanation for specific traits.
Today, we simply can't know: With the exception of genetically engineered organisms, genomes, and the traits derived from them that we observe today are the product of evolution. However, in many cases, we cannot know the evolutionary mechanism that gave rise to the trait, especially in humans: Was it adaptive, i.e. the trait was so beneficial to the population and/or its individuals that they were able to produce more offspring? Or did the trait get fixed in the population by chance? (note that in e.g. bacteria, with their enormous populations, we can quite safely assume that every trait we observe is selected for). There are two problems often faced when trying to answer this question. (1) You need a large amount of genetic information on many individuals in order to statistically determine how the trait has been propagated, and (2) in many cases (sickle cell anemia is a notable exception), we are not able to really quantify the fitness advantage of a trait, and/or we simply don't know where on the genome it is encoded.
Claim 2: evolutionary explanations are nothing more than "just so stories"
This claim is false in its general scope. Yes, as mentioned above, not every trait is adaptive, and not every trait is well enough understood so that we can make a claim. Thus, some "explanations" (which should usually be labeled speculation) may be "just so stories", though many of those are simply hypotheses that are not testable yet, at least not on the level of the genome. That is likely to change as sequencing technology improves, and the $1000 genome becomes available (see here for a bit of speculation what this will do for research).
However, there are plenty of examples of evolutionary explanations involving adaptive changes, even in humans, such as sickle cell anemia, lactose persistence, or the adaptation to low oxygen at high altitude.
EDIT 2: Not all "explanations" are equal. There is speculation, hypothesis, and what I would call founded explanation. In the OP and my above post, "explanation" is normally understood as hypothesis or founded explanation, rather than speculation. "Speculation" is what you can just make up by purely using your superior intellect, but without needing to consult actual data. Within the realm of evolution, speculation is often resulting in just-so stories, since many people tend to not understand how evolution works. "Hypotheses" are what you propose when you actually consider the data; they are ideally haven't-gotten-around-to-do-the-experiment, rather than not-yet-testable, and should not be sorry-can't-test-this-ever. Founded explanation is once you successfully confirmed a hypothesis. This may still turn out to be wrong, since reality, and biology in particular, tends to be more complex than what we think, but it's at least consistent with what we know right now.