Rich - is the 9-18 not that sharp? Are you not overly happy with it?
Happy with it so far, seems pretty sharp and contrasty. My only complaint really is the price. £500. I reckon the Oly 12mm would be £300 and offer equivalent sharpness.
After using the 20mm quite a bit I think I am leaning towards usage of primes more than zooms. I have a feeling that my trip next week will change my mind as I feel the 9-18 has great potential, I just haven't had the time to really use it.