I can give you a partial answer, I use a 0.5 reducer with my Atik when guiding with my TMB as a 1200 mm focal length for guiding is well to the north of stupid. This is a 1.25" animal and works well with the Atik 16 as it is a small chip. Once you start to increase the chip size you will suffer from an increasing degree of vignetting. My guess is if you use a DSLR with an APS chip, let alone a 35mm one, you will get the effect of looking down a length of pipe.

As regards are they any good, have a look at the price of some of the AP reducers, well over two grand.

I suspect your best bet will always be to use the scope at prime focus rather than afocally.

Dennis