I asked the same question you asked before getting this last summer, but now that I have it, I am glad that I did - it gets a lot of time on my GF1 body.
As Brian pointed out, this is a big, heavy lens. And with with the adapter, its looks like a super zoom when paired with a MFT body. The build quality is great and it feels very solid when paired with my GF1 (I personally like the heft). Since the GF1 doesn't have in-body IS, the weight can work against you though when shooting low light (especially if your hands shake like mine).
Here a few low light shots from July 3rd that I don't think my other lenses could capture. Candlelight and sparklers for illumination - wide open at ISO 800 between 1/25 and 1/40:
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/aBNHz8PJTjHZw9GEzoD8Dg?feat=embedwebsite">http://lh6.ggpht.com/_8rsKLcUXdBs/TTZMyKKPBoI/AAAAAAAADTo/zCCx-8oNpOk/s640/P1060894.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/YMmVgDqn4pQhSeW7Oqp2ag?feat=embedwebsite">[img]http://lh4.ggpht.com/_8rsKLcUXdBs/TTZNPo3v95I/AAAAAAAADTs/fd7ehLdKyDQ/s640/P1060969.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/IzVz1K35z2RmIJQ93HSRCQ?feat=embedwebsite">[img]http://lh6.ggpht.com/_8rsKLcUXdBs/TTZNldvme4I/AAAAAAAADT0/zQv8l9zvFT4/s640/P1070003.JPG" height="640" width="480" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
I find it super easy to focus on the GF1 - I have four children aging in range from 4-10 and I am able to use it to capture candid moments
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/GDz1kGtTWiuFj7nmqLEPEA?feat=embedwebsite">[img]http://lh3.ggpht.com/_8rsKLcUXdBs/TTZJZFl0hDI/AAAAAAAADS8/w0Ov4eajyMg/s640/P1030606.JPG" height="640" width="639" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/K2FGySyg1fW7HX3XRJtDvg?feat=embedwebsite">[img]http://lh5.ggpht.com/_8rsKLcUXdBs/TTZJFAe--qI/AAAAAAAADS4/I6idquKnw88/s640/P1020146.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/uk2u5BKYWSRG68SNHt4R8g?feat=embedwebsite">[img]http://lh5.ggpht.com/_8rsKLcUXdBs/TTZKs-JraHI/AAAAAAAADTM/fOJ3Bn_712Q/s640/P1070137.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/PPmBSSGMPhwBuczwnH1XaQ?feat=embedwebsite">[img]http://lh4.ggpht.com/_8rsKLcUXdBs/TTZJs4zy-ZI/AAAAAAAADTE/AnGjtSpFwiw/s640/P1060752.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/7SeNqsUYoQgfKe3PzW0EhA?feat=embedwebsite">[img]http://lh4.ggpht.com/_8rsKLcUXdBs/TTZSLQT0D5I/AAAAAAAADUA/4jpjfv9dVsY/s640/P1030659.JPG" height="640" width="480" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
Personally I like it a lot for portraits
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/3TgNVkhrlYc-l_ms8xXV8g?feat=embedwebsite">[img]http://lh5.ggpht.com/_8rsKLcUXdBs/TTZScAaysaI/AAAAAAAADUE/0mqrDfrEpTQ/s640/P1020262.JPG" height="640" width="480" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/5o_QQpVMyQQziXrcNTpxJA?feat=embedwebsite">[img]http://lh4.ggpht.com/_8rsKLcUXdBs/TTZStVxYKLI/AAAAAAAADUI/H9f-gMt69Js/s640/P1060689.JPG" height="640" width="638" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/THvSCq72oZ_wDDnAqNPjjg?feat=embedwebsite">[img]http://lh5.ggpht.com/_8rsKLcUXdBs/TTZLZPo6WyI/AAAAAAAADTY/04KRe9PdfGY/s640/P1070265.JPG" height="480" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
Although it also does good for still life too:
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/G5P28lw7qWBUrQSVFzIFdA?feat=embedwebsite">[img]http://lh6.ggpht.com/_8rsKLcUXdBs/TTZMdt8BvYI/AAAAAAAADTg/k8U7Hobzvp4/s640/P1060824.JPG" height="640" width="480" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/wgt0y88Ij_7EQ2LIdHN3ww?feat=embedwebsite">[img]http://lh6.ggpht.com/_8rsKLcUXdBs/TTZMlxDgyJI/AAAAAAAADTk/Tcgc7WQYLgs/s640/P1060836.JPG" height="640" width="480" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
By f2.8 to f 4 opens up nicely (focus on the faucet)
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/L9tJ1Ad5qHCKqaPi0a72TA?feat=embedwebsite">[img]http://lh3.ggpht.com/_8rsKLcUXdBs/TTZIuc3nNMI/AAAAAAAADSw/wLcTFES7O8M/s640/P1160586.JPG" height="428" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/Nikon50mm12?feat=embedwebsite">Nikon 50mm 1.2</a></td></tr></table>
And it is a good video lens too ...
<iframe src="https://player.vimeo.com/video/17464619" width="400" height="300" frameborder="0"></iframe><p><a href="http://vimeo.com/17464619">Water Main Break</a> from <a href="http://vimeo.com/stephengeis">Stephen Geis</a> on <a href="http://vimeo.com">Vimeo</a>.</p>
And as an added bonus, it works on your Nikon bodies too ...
(On an F3, shot wide open)
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/FxTXtKjTmvkavmVZ9Azfjw?feat=embedwebsite">[ATTACH=full]155922[/ATTACH]"424" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/AroundTheHouse?feat=embedwebsite">Around the House ...</a></td></tr></table>
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/0fHLyUwiZQJ-uvgIi0YI3g?feat=embedwebsite">[ATTACH=full]155923[/ATTACH]"424" width="640" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/StephenAGeis/AroundTheHouse?feat=embedwebsite">Around the House ...</a></td></tr></table>
All in all, one a solid, all-around lens, one of my favorites ...