If you're going to make a film of Porridge, this is it. The nature of film demands a "big" storyline, and what could be bigger than the plot of this?
Some people have said that the film doesn't quite have the same zest as the series. That is true - but then it goes to show how amazing the series was, because this is one heck of a good film.
It has also been said that the plot (which I won't give away for people who haven't seen it) is weak. Here I disagree. The only minus point I have about the plot is the fact that the subplot about new arrival Rudge disappears about 2/3 of the way in. As for the main plot, see above. It's the only story you could do when making a film of Porridge!
OK, so the pace of the film is a little slow - BUT you must remember that some people watching the film aren't familiar with the TV series, so they had to set the scene first.
They could easily have re-used hundreds of scenes and lines from the series, but to their credit, they didn't - both Fletch and Godber are nearing the end of their stretches, and they brought in a new "first-offender", the aforementioned Rudge - that way, Fletch is able to dish out a little bit of advice to him the way he did to Godber in the series - again, to ease in "first time" Porridge-viewers.
When you consider the number of TV sitcoms that were adapted for the big screen, and how much the quality varied, the good ones stand out all the more. And this is one of the best.