MOTs will give off low voltage (2kv or so) at high amperage, which makes them very deadly. But if you can get enough voltage to make a ladder keep on arcing, you get really fat, hot , long arcs which are pretty cool. If there's not enough voltage, the arc won't rise, itll look just like a welder.
But look out man, those things can kill. And beware that there are usually three leads out of the transformer. One is the HV line, the others I think heat the filament in the magnetron. The other HV lead is the transformer case itself! If you don't know that, it could kill you! It pays to be extremely cautios