Gun Disciple incorporates "realistic" abilities for a character that may have Strong-Soldier, or possibly Fast-Gunslinger predominant in their build, or even Tough-Bodyguard, for the culture it depicts is very much one of protective action. The key to understanding it - and finding it of any use - is the Colt 1911M1A1. If you love the gun, you'll love this prestige class. If you don't have any expertise with the gun, you will have to argue with your GM as to what qualifies as a "clone" of it because the author does not offer a list. A broad-minded GM might agree that your Desert Eagle is acceptable, in which case yippee, because you are on your way to becoming a living weapon of death-dealing. What the author does make clear is that modern autoloaders made with composite materials are out, out, out. So if you built your character based on the Glock since it's masterwork, fuggeddaboudit.
This prestige class offers what to me is a unique perspective. The class is the product of a single expert's translation of his own skill set into d20 Modern. Inevitably a single perspective may be loved or hated, endorsed or rebutted. If purchasers accept that this comes from such a perspective they will be the better for it and proceed to actually fitting it to their character or campaign.
Another reviewer has mentioned the high skill/feat requirements. However this class is so lethal I thoroughly endorse the requirements (with the caveat covered below). They do mean that you need to prepare well in advance - the Gun Disciple is not something you can just switch to.
This is a unique prestige class that adds directly to the d20 Modern opus. It explains where the authority for the class design comes from; it explains the class in the usual way; and lays out the talent progression. I also liked the background the publishers add to make it more than just a greyscale PDF. The only reason I'm marking it down from five stars is the uniquely restrictive weapon requirement and the grey area around what might qualify as equivalent to a Colt M1911A1.