This movie is a slightly extended version of the original play and adds bits to show more of Callan's sometimes vicious nature and Callan's and Lonely's more friendly interaction.
The performances of Edward Woodward and Russell Hunter as Callan and Lonely are up to their usual high standard but I missed actor Anthony Valentine in the role of Toby Meres as he did in the early TV series, he completed the strong triangle of main characters.
Whilst I think the film is only worth having if like me you get it for a very low price and as an introduction to the TV series the DVD does also include an excellent (one way) interview from Edward Woodward which in itself justified the price I paid.
I would strongly recommend Callan - The Monochrome Years [DVD]  [1967
with the original play and what remains of the first and second series an excellent DVD and good value, 11 episodes and the play, all good and interesting stories, good screen quality too and of course Edward Woodward's fine portrayal as the troubled assassin.
Personally I'll wait and see if the prices of the colour episodes sets drop before buying but I might give in before then as I remember some great Callan stories that I've not seen since early Sky satellite days.
If you like this high quality of acting, story telling and characters then I also suggest you look at another fantastic TV series and good quality DVDs with a great actor and character made for each other, Public Eye with Alfred Burke as Frank Marker starting with Public Eye - The Complete 1969 Series [DVD] [1965