I have a vetical forge as well- I use it for welding, forging and heat-treating. I have a thermocouple I can drop in and measure the temp quite accurately- there is almost no variation in temp except when you put a large, flat piece of steel in there and disrupt the flow... (which is what happens when you put a big knife in there...) That difference was only a few degrees. Enough to cause a slight warp in a blade on the quench, but simply rotating the blade every few seconds has eliminated that problem.

I agree that it is nice for welding- you can put crumbled refractory or kitty litter in the bottom and replace it when it gets too full of borax. Also have heard the theory that with the combustion taking place in the bottom of the chamber you are getting a more reducing atomosphere. It makes sense to me but I have no evidence save that I can run this forge very reducing.