DS did this along with his GCSE's. He enjoyed it immensely, as his aim has always been to join the RAF (successfully applied, enlists at end of August all being well!)
However, it was only equivent to 2 Grade D GCSE's at best. The course content was, in his opinion, perfect for Army candidates, over the two years he spent probably a quarter to a third of the course on army activities, including a 'look at life' residential course and various motivational days run by them, which were excellent and provided him with a multitude of Certificates for his record of achievement, all itemised for the topics covered eg leadership skills, orienteering, fitness, etc. His teachers were extremely supportive and motivational which IMO was one of the best things about the course!
Probably the next most thorough area was the police - they had several visits from various branches eg drugs dogs demo, etc. and visited our local police HQ.
He didn't seem to do much about the ambulance service or navy, some limited work on the RAF, a half-day visit to the coastguard. He has just mentioned to me that the course content is to some degree at the school's discretion - they had 1 ex- police and an ex-prison worker tutoring at their school.
He completed the course with flying colours (excuse the pun, RAF!!) and quite an impressive folder full of certificates and coursework, improved confidence and some glowing references for the RAF from his tutors.
I must add however, that the RAF recruitment staff did tell him that they don't actually formally recognise the pre uniformed services courses as they had no input on their content when they were devised, and therefore we saw little point in him continuing this at 'A' level - I think the school initially offered it but AFAIK it was withdrawn.