Quite a good adaption, but now a bit dated
I recently found a copy of this in my local charity shop and looked at it again after many years!
It's a fairly faithful adaption of the novel, with some great background scenery. Some of the acting from the children is pretty poor in places and their accents will sound out-of-this-world to most modern children!
The best child actor is probably Suzanna Hamilton (later appearing in the film 1984, and TV series like Wish Me Luck), who fits the character of Susan well. And Sophie Neville as the character Titty is suitably air-headed! Ronald Fraser as Captain Flint doesn't really fit the part for me - a bit of a let down.
For today's children I expect that the cut-glass accents will be a struggle, but may the strange idea (for nowadays) that children would be let out on their own for days on end without any parental interference might be fantastical enough to compete with Harry Potter!
I see that the BBC is planning to film (more than 1?) of the Swallows and Amazons series of novels in a year or so (I've seen it mentioned on the Arthur Ransome forum at www.tarboard.net). It will be interesting to see how they get on!