I have one for my 90 on the way. Along with a Disto 5 as well.
I've had very good experience when using HPF rings and the Disto 5. No idea of the absolute accuracy but it seems pretty spot on.
I realise this is a digital forum and I use film but for those interested here are some images; all tripod mounted:
http://rdubois.zenfolio.com/p285878686
Also for those interested, here I used the Alpa using zone focusing (Warning: there are many pictures); those with Alpa TC image 67 onwards.
http://rdubois.zenfolio.com/p574705449
The combination of the Alpa TC and Nikon D3s for very low light worked well.
The Alpa TC worked really well in difficult conditions (loots of dirt/dust/moisture). The only gripe I had (reported to Alpa) was that the finger grip is very slippery in hot humid conditions and when the hands get sweaty there is a risk of the camera slipping off the hands. I've had a new (walnut) grip made with far deeper indents for the fingers.
All in all though both in New Zealand (holiday) and India (work) this camera gave a very good experience. It is beautifully built, small enough to use as a street camera and the detail provided by the 60mm, f 5.6 Apo Helvetar is simply outstanding.
Dubois, December 2011