I've been to a couple of ports that required them, but they are only required for unescorted access into secure areas. In one case I gave them my FAST card and they held onto it as "good enough" and in the other case I was simply escorted to where I needed to go. If you have a FAST card and get issued a TWIC card, the TWIC is likely to have the same expiration date as the FAST card, since they are essentially duplicates of each other insofar as the threat assessment.
If you want a TWIC the fee is $137.50, but, if you have a FAST card you get a huge price break at only $105.25.
I don't plan on getting a TWIC, I'm just not at a maritime port enough that requires them. It's like the tranponder for Canada, I just don't go up there enough time for it to pay for itself. The HAZMAT fee is bad enough, considering Panther won't put placardable HAZMAT on a van.