I remember first seeing Digby, the Biggest Dog in the World when I was about 8 or so, and I loved it. Rewatching it 10 years later, it isn't as good as I remembered it being, but it is fun besides.
I do agree with everyone about the special effects, they aren't that great at all. In fact, even by 1970s standard, they are actually pretty bad. I also agree about Spike Milligan, with someone of his calibre he deserved more screen time and perhaps better material too, he wasn't bad as such, he just deserved more. Coming onto the script and storyline, the script does have a lot of funny moments but there are some grey areas as well, while the story has its predictable spots.
However, there is also some funny dialogue and some inspired sight gags. Also, I loved the concept, and the film worked fairly well with it. While the special effects aren't great, the cinematography and scenery are still pretty good, and the direction is at least okay. The acting is decent, Jim Dale especially is a lot of fun, however it is the big dog Digby who steals the show, he is very endearing. And I liked the characters, yes one or two of them are stereotypes, but they are at least likable and funny.
In conclusion, a decent film without being outstanding. 6/10 Bethany Cox