I bought the G1 version last year as a replacement for my Canon 17-40 f/4, which has a very low corner sharpness, and it was really a big leap forward. 15mm at f/2.8 combined with image stabilization is perfect for handheld night shots of still subjects. The large curved front element looks really nice. The lens is very big and heavy for a wide angle lens, but that is no minus for me, as I also love big and heavy cameras.
Using filters on that lens is quite complicated, but I do not use filters anyway. Software like Viveza allows you to achieve a look that used to be only possible with a polarizer in that past. Today you only need a polarizer if you want to get rid of reflections.