First, the practice tests themselves. While the material is in fact very thorough and does cover the same topics, it would be a lie to say that it's as closely aligned with the test as is advertised. Most math subject GRE's start off with (relatively) easy and routine problems, and generally build gradually in difficulty; on the other hand, SubjectMath.com's tests are designed so that almost every question needs a "trick" to solve it in any reasonable amount of time. Have to find a double integral under a plane? Gotcha! Just use the volume formula for a pyramid. Horrible Riemann sum? Gotcha! Just turn it into log(n)/n which goes to zero. Tricks are good to have, yes, but you don't need them for the entire test. In addition, the problems are worded in a much more verbose manner, with gratuitous notation and obfuscated expressions thrown in just to confuse you (reminds me of math competitions a bit). As a side note, the author is in serious need of an editor (e.g. "aspires to zero", "devisable by", "hight of 2") --- not that it takes away from the actual content, but it does take away a bit from the professionalism.
Also, after skimming through the online modules, I would say the only ones that are worthwhile are the Additional Topics and Advanced modules. There's a LOT of good stuff in those, especially the Advanced one. Most of the other stuff you can figure out yourself with a bit of Wikipedia searching for basic facts, or is already covered in the existing books. (Hint: Learn everything you can about eigenvalues and rings. Ever.
) And honestly, you can get almost all of the tricks just by going through the solutions at the back of the practice test books. Whatever the case, this is another reason I take issue with the amped-up difficulty of the practice tests, as if to make you think "oh no! The GRE is like this? I'd better get the online courses!" Feels somewhat dishonest to me.tl;dr: While the tricks presented are definitely valuable if you already have a high score and want to raise it by those elusive last few points, it's not really correct to say that the material is representative of the GRE subject test as a whole.