The best attributes of this lens are its tiny size, low weight (100g), and excellent low light performance. Its image quality is excellent and at f1.7 it's currently the fastest native m43rds lens that money can buy, with both auto-aperture and auto-focus.
The worst attribute of this lens is its noisy auto-focus motor. In my experience, the Panasonic 7-14mm f/4.0 Micro Four Thirds Lens for Panasonic Digital SLR Cameras and the Panasonic 14-140mm f4.0-5.6 HD (the GH1 kit lens) are both essentially silent when auto-focusing and they rarely hunt. This 20mm f1.7 lens is acoustically noisy, really too noisy to be used for movies unless you are going to record audio separately, and the autofocus hunts frequently *while taking movies*.
So all in all I'd say it is a great lens for still photos but not ideal for movies. Ultimately I recommend it for GH1 owners because at f1.7 it will allow you to take better pictures and movies in low light environments than is possible with the much slower f4.0-5.6 GH1 kit lens.
Update! This lens is cheaper and smaller than the Panasonic 25mm f1.4, but the 25mm is better than this lens every other way. If you want greater image quality or need silent continuous autofocus for movies, I'd recommend that you get the 25mm f1.4 instead.