Mark,

With gas, it's as adjustable as any stove I've used. The stove maintains an excellent low simmer. Take a look at the photos toward the end of the slideshow on my blog for simmering photos on gas.

With white gasoline, the flame is more adjustable than either of the Whisperlites currently sold, but it doesn't really simmer.

I haven't tried simmering with kero yet, but I can't imagine that kero will simmer any better than white gasoline.

If you're looking for true simmering, either run the Whisperlite Universal on gas -- or go buy a Dragonfly.

HJ