I'll start using it tomorrow for one reason: three STRICT tables feature set will very likely get gradually expanded with time and I believe it's also very likely that sticking to the new syntax will thus net increasing benefits with time.

Of course, migrating current databases is another story. They have plenty of CHECK constraints and app-level validation already so the value in migrating those is small (although I'd still do it because the ANY data type columns will not coerce / convert anything when the table is STRICT which to me is still a win).