Thread Magic is a thread conditioner, it makes the thread smoother so that it is less likely to snag, tangle, twist, or cause other friction problems.
It is similar to using beeswax although, I find this conditioner much better because it helps me to sew for longer without experiencing the discomfort I often feel in my fingers and wrists when hand sewing for a long time.
The product itself is the same as Thread Heaven however, the container is different. Thread Heaven is a small square that you need to push the thread into, whereas Thread Magin is a circular container with small indents in the edge that make it easier to draw the thread through the conditioner.
In order to test it for this review, I used Thread Magic for a couple of small projects. Firstly, I was mending a sheepskin glove for a friend:
The Thread Magic allowed the thread to be pulled through the leather much more easily than without.
Next, I added some small hand stitches to a cushion:
The final test was with tapestry wool and thick wool felt. I have been sewing these felt flowers together for a while now, I just do one or two every now and then.
Using the Thread Magic made it much easier, as the wool usually drags through the felt with a lot of friction, but once conditioned the wool pulled through very smoothly
I have not noticed any alteration to the appearance of any stitching whilst using Thread Magic, or Thread Heaven, but I have not used it on white, and although I doubt you there would be any discolouration, I think it would be wise to do a test piece first.
In conclusion, I think Thread Magic and Thread Heaven are both excellent, I recommend using them because they make the hand sewing experience so much easier on your hands. It is possible to use the conditioner whilst machine sewing, but I have never had cause to. Thread Magic is easier to use than Thread Heaven because of the container and even seems to have a slightly smoother formulation.
If I did not like using this product I would not give it this good review.