Inverse Volumetric lights have been updated in the 6.1 release. They are cleaner around the edge and render much faster than before. Edge anti aliasing is also very much improved.

Very Important Tip: You will get far better and much faster render results if you change the default shadow setting from 250 to 750. A low shadow setting causes more streaking in the volumetric haze and considerable slows down the render, by as much as five times! Start with a shadow setting of 750x750 and tweak the sample rate to adjust the cleanness (streakiness) of the haze.

Also, comparing results with those obtained in version 6, I found that you have to dramatically increase the brightness of the volumetric light (by up to 10 times) to equal the brightness level you got by default in 6.0

MV's Cinema 4D