It's (admittedly) a while since I last watched the movie, but from what I recall the dialogue and on-screen action can at times seem 'cheesy' , but it's very much in line with the old serials of the 30's. For me, it's the closest attempt at creating a comic book in cinematic form; the humour, the action and the clear definitions between good and evil.
I though that a more serious Flash Gordon movie would be a great idea, and all the pieces are there for someone to come along and write it. Same with Dan Dare.
The music, the costumes the actors all seemed to blend together to make a perfect movie, and I remember when (as a child) I went to see this movie at the cinema I left the cinema possibly more impressed than when I had been to see Star Wars. Even today, I think I would be more excited to meet in person Sam or Brian than say Mark Hamill or Harrison Ford. Not to say FG is a better movie, but the blend of continuous action, music and bright colours left more impression on me as a child than did the more sedate SW.
I've seen Sam Jones giving talks at some conventions, and I also went to one with both Brian and Sam on stage at the same time. I get the impression that Sam's off-screen personality is not much different to his on-screen portrayal of Flash. Both Brian and Sam were keen to point out the very positive messages that Flash Gordon the movie tries to get across, and it was great to see that whilst some actors are embarrassed or prefer not to talk about some of their more popular roles, these two guys really were just as enthusiastic (and still prepared to appear in a sequel!)
If you haven't already seen it, I highly recommend watching the movie 'Life After Flash', which is largely Sam Jones and Brian Blessed talking about the movie.