I tend to turn on subtitles when they're available, because I wouldn't understand all the speech in a film, unless the film only features middle-class characters speaking standard British English, which, to be honest, might make a rather boring film... Even a British film needs some Glaswegian psychos...

It's interesting to see how much the subtitles diverge from what's spoken. Sometimes you can see they've apparently just simplified it a bit, not losing anything important, but sometimes the differences seem gratuitous, or somewhat creative. Do the subtitles come from the original script, perhaps, while the lines were adapted by the actors on set without anyone updating the (tran)script?