The best thing about iMovie '11 stills is that they now match perfectly with the video frame, so freeze frames are identical to the preceding video frame. Likewise with stills from an Event. This exact match was how things worked with very early versions of iMovie, particularly version 2. But, I found that stills from iMovie HD 6 weren't as sharp or clear, or as saturated, as the video frames they were extracted from.
So, I much prefer the stills from iMovie '11 when adding them to a project. If I need them for other purposes (which I frequently do), I use MPEG Streamclip or otherwise a screen grab in full screen view. With screen grabs, the best approach is to create a new project and add stills from your Event, with each still having the shortest possible duration. Then show that project full screen and grab the stills from the frames using the key combination Command>Shift>4. Use the left/right arrow keys to progress through the project. I get excellent quality stills this way and generally import them to iPhoto for further enhancement, cropping etc. After processing in iPhoto, I often use them in Keynote, then import back to my main project in iMovie '11 for producing a highlights slideshow.
John