I think the answer really depends on the quality of the Breakfast club/Aftercare provision.
Ours is excellent, and DS has been attending 5 days a week since reception (Aftercare only - I don't need Breakfast club). There is a really good choice of toys/games and free choice or structured activities. DS has had the opportunity to play with children from all different year groups when, in Reception, they were very much isolated within their own playground at playtimes.
The children get to choose, a week in advance, what the tea menu would be for the next week. They get a light tea - such as soup/bread and beans on toast. 2 1/2 hours care costs £7 which is substantially cheaper than a childminder and siblings get over 50% discount.
They also provide full day care for Inset days which, as a teacher, is invaluable.
I think, however, that the quality of Aftercare is very variable, and I would recommend talking to the Aftercare co-ordinator to see what is offered. Also, I think some children would prefer a 'quieter' environment when very little.
For me it was a no-brainer - particularly as the school is opposite my house whereas the nearest childminder was 1/2 mile away (and I don't drive).