If you want to pull frames quickly and easily from your video clips, then you should take a close look at MovieDek from Limit Point. MovieDek is a video utility that'll scan a video file and export a range of frames as stills in just a few minutes.
MovieDek is a bare-bones app that launches with a small dialog box in the center of the screen. The app instructs you to drop a movie file on the application or the window to start the extraction process. There's also a preferences button and a quit button if you want to shut down the app. Inside the preferences, you can set the scale of the exported images as well as the image file format and image quality. You can also select the range of frames that you want to export.
Once you configure your settings and drop your movie on the app, MovieDek will start exporting your frames. The app works through the movie frames one by one, so it may take some time if you are exporting all the frames in a long movie. On average, I was able to export about 300 frames in less than five minutes.
I use MovieDek on a regular basis to grab the perfect shot that I missed because I was filming video of a scene instead of taking photos. The exported stills from a 720P HD movie are good enough for web usage, but I wouldn't plan on making a large print from them. Pulling stills from a video won't replace using your camera for shots. It's meant to be used in a pinch for creative uses.
MovieDek is available in the Mac App Store for US$9.99. You can also download and purchase the app on Limit Point's website.