Thankfully we visited these gardens on a hot day so the guided tour of the fountains became great fun especially with families. The guide knew exactly when to operate the fountains and made this part of the tour very entertaining. If you don't want to get wet then watch for the drier bits of ground. The guide was also sympathetic to anyone with cameras etc and hinted as to where to get out of the way.
Getting there is easy enough - there are bus stops outside (single adult fare was 4.80 Euros) straight from Salzburg centre. Anyone with a wheelchair can get round the fountains - there was at least one wheelchair user when we went.
There's a good cafe in the courtyard - the ice cream is recommended.
I'm definitely not a Sound of Music fan but lots of people went to the pavilion where some of it was filmed and seemed to return satisfied. The gardens are not spectacular away from the fountains but this place is worth a morning or afternoon especially if you've children with you.