right. read the doc beforehand, and for financial data use the time proven relational dbs. they're "slow" for a reason.
for massive user generated content where big numbers of statistics come into play, and noone would die if you lose their hard-composed 140 char tweet, use transaction- and lock-free, easily-sharded, rarely flushed to disk stores.
If MongoDB makes it possible for you to do something unique, that no other proven solution would allow, it might be worth the risk.
If you used MongoDB because you heard it's the latest hip technology, and your company suffers, you made a stupid mistake that you'll hopefully learn from.