I've just machined some Kennions wheel castings (Rob Roy) for a friend and they were very good, nice and crisp and very sensible machining allowance. I built a R&B gas engine from Polly castings a few years ago and was also very pleased with them.
Like Dave Smith above I am building Don Youngs Aspinall and have to agree the castings are very poor, too little machining allowance in some places and lots of filing and cleaning up on the wheel spokes, the wheels could not be finished to size, too little machining allowance. The cast iron used seems to be recycled drain covers.
I've just had to silver solder blocks on the motion plate as that was undersize, as were the tender axleboxes. Don't say I should have returned them that opens up a whole new can of worms! A shame as I like the designs of Don Young and the castings are only available from one source.
Next loco will be incorporating more fabrications and hacking from solid.
Dave