Well, admittedly the first time I watched it I thought the film itself wasn't the most riveting film I have watched, however Billy's performance is truly mesmerizing - Billy is exactly how you would expect him to behave in everyday public.
However I have since watched the film at least 8 more times in the space of 12 weeks and each time, I still thoroughly enjoy Billy. The film itself continues to grow on me and the storyline is cracking (this coming from a legal background!).
If you like Billy, buy it. If you aren't a huge fan, then rent it before buying it. It you are in a passing phase of Billy Connolly comedy, don't buy it because it will not meet your expectations of a great film.