In my LEA, the Catholic secondary schools are very good, but it is a bit of a chicken and egg situation. The schools are good, so the parents want to send their children to them, so they stay good.
There is a girls school in my borough which is excellent. Nearby there was a Catholic boys school, which is in the next borough. Traditionally, the children at the two schools were from the same families (brothers and sisters, similar intake). Both were grammar schools which went comprehensive.
The girls school kept standards high, mainly by streaming, and having high expectations of all students regardless of deprivation. The boys school went down hill. They did not keep the discipline standards up, and made too many excuses for family background. Consequently within families, girls would go to the girls Catholic school, and boys would go anywhere else. To put in context in 2006, the boys school got less 21% of its students 5 GCSE's including maths and English, its best results ever (in 2003 it got 5%). The girls school got 55%, a poor cohort, results were normally 65%.
Needless to say, the boys school is now a mixed academy, and is improving, but from a very low starting point. Last year, it got 43% of students 5 GCSE's including maths and English, but the girls school got 80%.
So, rather like any school, Catholic schools can be good or bad. Things can change.