PostgreSQL vs Mongo

All;

One of our clients is talking about moving to Mongo for their
reporting/data mart.  I suspect the real issue is the architecture of
their data mart schema, however I don't want to start pushing back if I
can't back it up.

Anyone have any thoughts on why we would / would not use Mongo for a
reporting environment.

what are the use cases where mongo is a good fit?
what are the drawbacks long term?
is mongo a persistent db or simply a big memory cache?
does mongo have advantages over Postgres hstore?
etc...

Thanks in advance...

/Kevin