their initial lens was de-centered so not great for astrophotography but a 2nd lens was pretty good in the corners with only mild astigmatism making it a very good lens if you get a good copy
quite susceptible to flare and ghosting
practically as sharp and optically as good as the excellent f/2.8 Olympus 7-14mm zoom even at f/2.0
some barrel distortion but overall it is well-contained
traces of chromatic aberrations especially along the edges and in the corners