So, when is cleansing worthwhile? It says it can clear away temporary background count then follows it up with a really vague definition of temporary vs permanent background count. It goes on to mention that as long as the source of the background count is there, the counnt will come right back. I can see it clearing away the emotions of a murder scene, for example...

But if you tried it in a toxic domain, do you think the count would immediately spring back full force? Would cleansing just fail? Or would the background count "flow" back at a certain rate? Like +1 BGC / hour till it's at full force... or maybe it's just worthless.

I'd say it'd flow back over time.
For toxic domains, you have to clean up the environment and then cleanse the cuont. This goes way beyond celebrities getting cozy with oil slicked ducks and some moist towelettes on camera. This means digging, difting, removing stuff that was best left never made in most cases.

In a hotspot on a run the slightly shorter time might mean the difference between leaving signatures... or not.

Erasing signatures takes a number of complex action equal to force of spell. SR3 p.172

Cleansing takes a number of complex action equal to twice the background count level.
hmm... "Cleansing backgroundcount also erases any other astral traces and signatures associated with the background count." MitS p.74
I would rule that it erases normal signatures as well.... (or do I have to murder someone for it to work?)

also taking into account that erasing has to be done on each signatur seperatly whereas the cleansing can be done on all signatures in an area in one go (I presume)

This is more useful for really stealthy, Shiawise style runs than for classic Ares actions. It is also really useful for NPC magicians who have to clean up after a runner team. People like Lone Star and corporate damage control teams.

or SR style "ghost hunters" dealing with the paracritters and threats for the norms.

No it is not useless. GM's can use it to clean up the Background Count of an Area or reduce the Background Count Modifiers for the Cleaning Magician's own Spell Casting, Conjuring, and Astral Abilities.

Cheers

QM

Cleansing has its uses, but for shadowrunners it's significantly outweighed by most other metamagics. I was significantly annoyed when they made it a prerequisite for filtering. There's a much more useful metamagic for a shadowrunning mage.

I think it's insanely useful for mages, as it allows the mage to clean his astral signature from a crime scene. You can't beat that. It's very popular in my games.

Cleansing is great for PCs that do not want to leave an astral signature. Use it after you spammed spirits or spells, and you don't have to worry about changing your astral signature every so often. Unless you plan to have your astral signature known to every police department in the world, you would take this metamagic.