It's a controversial topic.
Ned is correct that most of the holding tank products being sold consist mainly of a masking fragrance. I use Aqua-Kem, which contains formaldehyde and methanol, which have a bacteriostatic effect that reduces the stink at its source. However, it is toxic.
I find that there are situations where I end up boodocking where I have a means of disposal of grey water (usually by pumping it and hauling it) but not sewage. Odor control then becomes a more serious problem than it might otherwise be, particularly in hot weather.
When camping in full hookup sites where it is possible to use all kinds of water and dump tanks frequently, the chemicals aren't necessary.