Have not tried biomagic but any toilet fluid that does not contain formaldehyde will do pretty much the same thing(and smell nicer)
There are adverts on fleabay and the like that sell "GREEN" toilet fluids at a very reasonable price.
I have used biomagic in our Thetford cassette toilet but only along side a home-made SOG system. Works very well and doesn't have that terrible smell of the more chemical based liquids. Not sure how it would work without the SOG though as this takes care of most of the decomposing.
I have heard that some of the green fluids don't work very well but I did a google search and found some good reviews for Boimagic on the motorhomefacts.com forum so I have bought some. I will post a review once we have tried it.
I use Biomagic,have done since we bought the van from new.It should only be used in a clean cassette,ie:one that hasn`t previously been tainted with chemicals,or it wont work.In a clean cassette,it works very well IMO.
We have now had a chance to use it and we were very impressed. We stayed at a site in the New Forest with minimal facilities. It worked really well - no smells at all, and definately starts to break down the waste. The fluid itself has a pleasant mild smell, much nicer than the Thetford fluid - I got a free sample of Thetford green with the porta potti, and I found the scent unbearable even without opening the bottle.
if you get a chance have a talk with the guy who sells biomagic (Alex), he's a real enthusiast for the stuff and will tell you how widely it's used for other applications in the rest of the world.It's great stuff.