Wikipedia states it pretty well, "When factor analysis (a statistical technique) is applied to personality survey data, it reveals semantic associations: some words used to describe aspects of personality are often applied to the same person." [1]

The Big Five are a statistically-derived groupings of phrases that are reasonably stable when measured from different angles. I think it would be fair to call it a "modest initial result" if compared to the physical sciences, or "wildly successful" if compared to basically anything prior that came out of the social sciences. There are some quite reasonable critiques listed on the wikipedia page, such as a lack of modeling theory, or only accounting for a portion of personality differences, or arguments for a slightly different number of factors, etc. That said, it seems to be one of the first actually solid findings that ever came out of the social sciences. It's like the whole field has been flailing around, swept down a river in the dark, and for the first time we found a stone that has a bit of purchase, even if it's covered with moss and slippery.

https://en.wikipedia.org/wiki/Big_Five_personality_traits

Wildly successful is a term that would be better applied to item response theory models, the Big 5 (and personality psychology more generally) are at the leeches stage compared to them.

That being said, it's much much harder to develop measures when you rely on other measures of the same type to validate them, which is where personality testing is right now.

By response theory do you mean behavior research by Skinner, Panksepp, etc? Yes, basically everything in personality psychology barely even registers compared to that, I agree that my comparison was too broad. A more reasonable statement might have been that Big 5 is wildly successful compared to other personality models.

Thank you for the correction.

Nope, I meant item response theory: https://en.wikipedia.org/wiki/Item_response_theory

It's a much, much better method for designing and analysing questionnaires, and it is the foundation for all standardised testing (SAT, MCAT etc).