Having the "Cowboy Bebop" tag in front of the movie's title is both a blessing and a curse for the movie. The blessing is that it gets to use the great foundation set by the series. In particular, it's always good to see the colourful characters that make up the crew of the Bebop once more. I've grown so fond of them it's like meeting old friends again - they've not changed a bit. This movie is good in much the same way that the average single episode from the series is good - it's just a really well made action/adventure story.
At the same time, it's only natural to start comparing this movie against the series... and this is where it falls considerably short. You see, the series is far greater than the sum of its parts - although each individual episode is very good in its own right, they are rarely spectacular with a few notable exceptions, and this movie ain't one of the exceptions. When you add all the episodes together, then you start seeing what makes "Cowboy Bebop" so brilliant - its variety. When you take this aspect away, the whole idea starts to break down. The "Cowboy Bebop" movie is supposed to feel like a longer story, but instead feels more like a normal length story stretched out to be film length - perhaps this is due to the fact that I've come to expect a certain amount of freshness from the series' constant genre switching, and obviously this freshness is lost when it comes to this movie.
Still, most of the essence of "Cowboy Bebop" is still here, and it's enough to make "Knockin' on Heaven's Door" a very good movie in its own right... but it just can't match the enjoyment and addictiveness offered by the series. As the final outing featuring the crew of the Bebop, it felt like a bit of an anti climax because it couldn't quite compare with the grand finale of the series. So I would advise other people to watch this before episode 23 of "Cowboy Bebop" and allow the ending of the series to finish the franchise in style.
