I have been using bismuth here for almost 3 years. I wanted to give it a fair shot for the simple health and regulatory reasons. I have found the following;
1.) You can't cupel at the same temperature as for lead, you need to be about 1580-1600 degrees F.
2.) For standard ores, say less than 0.5 opt Au. As long as you are careful with temperature on cupellation AND the atmosphere in the furnace. It can be a direct replacement for the lead based flux. But be careful to run more frequent standards.
3.) For higher grade concentrates and bullion, I will not use bismuth, without perfect conditions that most labs cant achieve, you will have significant losses. We have been trying to be very careful when doing the SMA round robins(to check the bismuth assays against others using lead), but the trend is for the bismuth you will have a lower assay.
4.) I have found that the bismuth trioxide in the flux is VERY destructive to furnaces, and furnace elements. I have found a way to reduce the destruction by simply lowering the furnace temperature to about 1850 and fusing for 70 minutes, but the destruction does continue and I have not tried the lower fusing temperature on higher grade samples as yet.
Bizmuth is 5-6 times more expensive than its lead flux counterpart, but it is safer to use (so far) and waste can be thrown out in the regular trash.
However, for reasons mentioned above, in the future if I had the choice, I would not use the bismuth flux at all. (Sorry Dave)