Testosterone Decanoate is available under the trade name "Neotest 250" by the Mexican drug company Loeffler. Most people will have used testosterone with this ester in the popular testosterone blend "Sustanon 250", so it's far from something new on the market. It's just never been offered outside of a blend until a few years ago.
Basically, like all forms of esterfied testosterone, what we're looking at is the parent hormone bound to a fatty acid (the ester) to delay its release into the body. With regards to the decanoate ester, this gives us a nice long active life in the body, and there's no reason to inject this more than once every week.
As with all forms of testosterone, this product will cause weight and strength gain, fat loss, a libido increase, an increase in IGF-1 levels, Red Blood Cell count, and of course increased recovery from training. Again, as with all forms of testosterone this product is subject to aromatization as well as 5alpha-reduction, which are the pathways through which testosterone is converted to estrogen and dihydrotestosterone respectively. Excess build up of estrogen can be responsible for gynocomastia (development of female breast tissue in males) as well as bloating and water retention. Dihydrotestosterone can cause both acne as well as hair loss.
As with all forms of exogenous hormones, using this product will shut down your body's natural production of testosterone. In addition, cholesterol, immune function, and several other health markers may be adversely affected.
From a very practical standpoint, I've got to say that the main advantage to using something like this product is that you can stack it with Deca-Durabolin and inject them at the same time each week, and not have to worry about one hormone decaying at a different rate than the other.
Again, from a very practical point of view, I can't see why this stuff isn't more popular. I have literally never heard of a single person using this product, but if it can be purchased for $100 or less for a 10cc vial, then would wholeheartedly recommend it.