Due to the depth of field fixed at f8 focus is not really an issue or a problem. Set it up to infinity and everything from 6 feet and up will be sharp.
Center filter makes it really slow (f16) which is horrible. But I guess you can live with that. Tripod is not necessary but recommended. Since if you are planning to use Velvia you shutter speed will go to hell on a not so sunny day.
I still haven't got a chance to really use that lens because 21mm seems wide enough but here's a test shot (hehehehe):
As you can see fall off at the corners exists since no center filter was used in this shot.
If you want to go wider and faster there is a lens from Voigtlander 12mm f/5.6 or 15mm f/4.5 also in manual focus that can be adapted to G2 by company called Zork. If you go here you can see more info in that pdf on page 5:
There is not that much fall off on the 15 (although some exist) as opposed to 12mm but they are reasonably priced alternatives to Hologon. There is no center filter available for Voigtlander lenses. I've used their glass (including 12 and 15) on my screw mount equipment and quality and sharpness is superb. The only thing is that you can convert the lens by Zork and pay $1300 or you can buy a lens in original screw mount version ($700) and but a Voigtlander body ($100) and have a setup dedicated to extreme wide angle photography.
As a world of advice. Cap on 16mm has a tendency to become loose. I used little gaffers tape to make sure it gives me a tight fit.
