I've only use negative lights once, to dim the corners of a room. Problem is, they reflect darkness on other objects, and it can look weird. But as you say, they are OK for simple scenes. I find a huge slowdown occurs on soft lights. My last pic rendered in about 20hrs, with one soft light out of 35. Originally, I had 4 soft lights, but it was taking 5 minutes for each line of pixels on the first pass (about 10X as long!). It's actually quicker to use, say, 20 normal lights grouped close together, than one soft light. I hope they make it a bit speedier in Bryce6, 'cos soft lights really do look cool.