A lot of opposite viewpoints on here -- it's great.
My experience: I did CCM fellowship because a) I like CCM, b) I wanted to be able to do CCM in practice, and c) I could afford (literally -- opportunity cost) to do it. No kids and not tons of debt at the time.
I'm not doing CCM currently, but having done the fellowship opened doors (not closed them as was others' experience) due to presumed facility with sicker cases etc. It gave me time to get Basic TEE certified. It got me lots of experience with difficult patients, difficult families, difficult procedures, conversing with surgeons professionally and personally, etc.
There are a lot of good reasons to do CCM, but making more money in the ICU than you would in the OR isn't one of them.