Hi Angus,
I recently bought an Astronomic CLS filter for my 60d. The sales person told me not to bother pointing it at Globular Clusters or stars as the colours will be out of whack. It should only be used on emmisions nebulae etc.
I've also wondered wheather I could use it to point at Galaxies but I think not as it will cut out too much light.
Here are my results of the Tarantula with (red nebulae) and without (blue nebulae) the CLS filter. Both were taken from my balcony in Carlton under heavy light pollution.
Post processing was a little painful because everything came out RED RED RED, I was suprised and a little disapointed but others said that my results were good, so it beats me
