Journal of Cosmology and Astroparticle Physics

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Continuing my research into the current state of journals which publish
papers in astronomy, I came across the Journal of Cosmology and
Astroparticle Physics (not to be confused with the Journal of
Cosmology!). While most traditional journals now have both a paper and
an online version, I don't think any major journals have dropped paper
completely in favour of being online-only; it's probably a tough
decision to make. The JCAP appears to have been online-only from the
beginning, which makes sense in today's world: the advantages of a real
journal without the baggage of paper. It is run by serious institutions
(SISSA and IOP) and has sensible people on its advisory and editorial
boards. Interestingly, it also has no page charges but is apparently
supported by subscriptions, which are reasonably priced.

How do readers who are familiar with all of them rate its QUALITY as
compared to ApJ, MNRAS and A&A?

I was a bit surprised that its impact factor is larger than ApJ, which
has the largest of the "big three" (though not that much larger than the
other two). Is this due to the subject matter (more particle physics)
with a different citation culture or to some skewing of the impact
factor. (The impact factor is basically the average number of citations
per paper in a journal. I don't want to discuss its pros and cons here,
but just understand why a relatively obscure(?) journal has such a high
impact factor. (A similar journal with an even higher impact factor is
Living Reviews in Relativity, but I suspect that this is due to the fact
that journals devoted to reviews (and review articles in other journals)
tend to get more citations.))

Like ApJ (and AJ) one has to transfer copyright to the journal (one
doesn't in the case of PASA and MNRAS; A&A I'm not sure); I assume that
JCAP doesn't object to posting on the arXiv. Since the arXiv solves the
distribution problem, main considerations when deciding where to publish
are the reputation of the journal and the costs involved.