Pacing is pretty important to a film, but I don't think that bad pacing always ruins it. It's really a matter of perspective as to whether the pacing makes or breaks a movie. Good writing, acting, and or direction can usually make up for a movie dragging or going by too quickly.
A movie series that shows all sides of this equation would be the Spider Man Trilogy. My favorite will always be the second movie, but the first movie's pacing is far better. Pretty much everything was set up and flowed well from Peter's status in society to his crush on Mary Jane to his being bitten by the spider; gaining and learning to use his powers, making money off them and so on. Nothing about the first film made me think "Can we get on with it?"
The second movie though, as much as I love it, dragged a bit and had some padding. For example, most of what transpires between Peter dumping the suit and Doc Ock attacking Aunt May could have been cut and it would have made the pacing much better. But the sequences that do drive the plot are so well written and directed that the padding doesn't bother me that much. In a lesser movie it probably would have been a dealbreaker though. Again a matter of perspective.
The third film's pacing is also off, but in the exact opposite manner. Everything goes way too fast and important events go by without a proper transition or explanation. Unlike the second film the writing, acting, and direction don't compensate for this because it's all terrible.
A more recent example could be the Dark Knight. A great movie whose pacing is also a bit off. As awesome as it was, the Hong Kong sequence was padding and could have been cut without affecting the plot in anyway for example. But the direction, writing, etc in the movie makes up for any pacing issues it may have.