Around 400 Year 6 pupils from English-medium primary schools took part in the five and six-week Welsh courses. Estyn found most made significant gains in their language skills. Some pupils from northern counties even opted to go on to Welsh-speaking secondary schools afterwards.
However, the agency said the use of non-fluent teachers in some areas, mostly in the south, should be reviewed.
It also concluded immersion courses could not offer good value unless a "bilingual curriculum is sustained". Inspectors suggested that English-medium secondary schools should make some subjects available in Welsh to allow pupils to use their language skills.
The findings have upset the leaders of a pilot scheme in south Wales, who say comparisons are not possible between the north and south.
John Williams, school improvement officer for Rhondda Cynon Taf, said the aim of the county's pilot project was to help less-than-fluent teachers get up to speed in the language of heaven, alongside pupils. Just one primary school, with 84 pupils, was involved.
He said: "What Estyn doesn't seem to acknowledge is that we do not have enough qualified teachers in the county to take the courses. The idea of our intensive language project was for teachers, as well as pupils, to learn Welsh.
"I am all in favour of bilingualism. But our geography dictates that the more Welsh-speaking areas will have a head start.
"In RCT, 85 per cent of parents decide to send their children to English-speaking secondaries."
Small primary schools in Denbighshire were also seen as weaker pilots, due to a lack of resources and their rural locality. Schools that managed to teach 90 per cent of the curriculum in Welsh, often larger schools in the north, were seen as the most successful.
Estyn recommended the Assembly government should further support the models producing the best results.
It also called on the Welsh Language Board, local education authorities and secondary schools to offer extra-curricular activities in the summer holidays to ensure further development of pupils' linguistic skills.