The Panasonic Lumix G 42.5mm f/1.7 ASPH Power OIS is a showcase of what's so special about the Micro-Four-Thirds system. Of course, it's tiny and light-weight but it's also very good optically. And this is a general theme across the majority of Micro-Four-Thirds lenses pretty much regardless of their price tag. The center resolution is very sharp at f/1.7 already and the borders/corners are no slouch either here. Stopping down improve the quality a bit further. Image distortions are already fairly low in raw files and negligible in JPGs. Lateral CAs are a non-issue. There's just a bit of light falloff at maximum aperture and it's gone by f/2.8. The quality of the bokeh is good overall but it's not flawless. The rendering of out-of-focus hightlights is impressive. However, the background bokeh could be a little smoother in difficult situations at least.

The build quality is on a high level as well. Due to its feather-like weight it lacks "substance" but objectively there isn't much to complain about really. The body is made of metal and the focus ring operates smoothly. You may miss some sort of weather-sealing though. The AF is quick and noiseless. The image stabilizer works nicely with a possible gain around 3 f-stops.

The pricing feel a little steep relative to the "little amount of hardware" but it's fair considering the quality of the lens.