A lot of people don't trust the CBO - and it's because it is so often wrong. Some examples are presented where it gets it wrong.
Of course being flawed doesn't mean its data isn't worthwhile - as its recent estimate on Republicans' repeal effort shows it will among other things repeal penalties on employers as well as on individuals.
Also worth reading is this optimistic epistle on Obamacare's repeal.
UPDATE, August 17: The CBO's math used for Obamacare turns out to be fraudulent.