When I say skeptical what I mean is that I have no doubt the measurements are accurate (as in not fabricated); however, I wonder what the full context of the benchmarks are and whether or not it's representative of situations like mine where the built-in parser started to fall over so we replaced it. It may also be the case that the PyPy parser would also beat ujson in this case, but if we are limiting our test case to just those where the CPython implementation is marginally faster than ujson then I'm not sure it's that useful a benchmark.
I too cannot comment on the quality of the implementation or the relative performance against other parsers...but it solved a performance problem for at least one project of mine in the past.
But the difference is pretty insignificant compared to Node and PyPyFull.