The history of the G Spot is an interesting read: it's the story of a whole bunch of male researchers who, after years of study, realized they didn't understand female anatomy. It's also a story of otherwise staid and phlegmatic white coat Ph.D.'s getting rather ugly and jumping into the cage for a little laboratorial face thumping. The whole story even includes a dozen cadavers along the way.
And that's good, because reading about female anatomy in the research can be a little "dry" shall we say? Consider this ground-breaking, mouth-watering description of the clitoris: "The clitoris is a multiplanar structure with a broad attachment to the pubic arch and via extensive supporting tissue to the mons pubis and labia. Centrally it is attached to the urethra and vagina. Its components include the erectile bodies (paired bulbs and paired corpora, which are continuous with the crura) and the glans clitoris. The glans is a midline, densely neural, nonerectile structure that is the only external manifestation of the clitoris".  I've included the reference in case you want to do a little reading.
Actually, this journal article is trying to point out that the clitoris is actually a much more complex structure than was initially realized. Us guys - we're simple. We're easy. But females are just as complex physically as they are in every other way!
The G Spot is no different. Researchers in 2001 stated that the G Spot was a complete fabrication because it could not be located.  Another researcher actually wrote a paper entitled "The G-spot: a modern gynecologic myth".  One team even noted that vaginal stimulation caused blood to rush to the clitoris and thus a vaginal orgasm was really a clitoral orgasm.
Case closed, right? Well, the next year a researcher examined fourteen cadavers and found that 9/14 had Skene's glands and 12/14 had erectile tissues in the general vaginal G Spot area. (The Skene's glands are responsible for female ejaculation and are the female version of the prostate.)  In other words, most females examined had analagous structures to us males within the vagina itself. (The 2001 researchers were strangely silent thereafter.)
Several years later researchers examined the G Spot areas of women who said that they had vaginal orgasms. All of the women had a thickened G Spot area or "urethrovaginal space". The researcher did further investigation and found that only women that had this thickened area were able to achieve vaginal orgasms. 
This researcher, Emmanuele Jannini, is the king of the G Spot and also found that PDE5 was mostly clustered around the G Spot, especially in women that had an thickened G Spot area by the way. The significance of this is that PDE5 is the same enzyme that Viagra works on in males. (PDE5 actually depletes Nitric Oxide.) In other words, most women have erectile tissue right in the area of the G Spot confirming what many enthusiastic males have been saying for years: this area can become engorged.
So what does all this mean? Well, there are a couple of important points. The first is that us males can actually learn something about females after years of intense and painful effort. The second is that it appears that some women, although no one has a hard - no pun intended of course - number, will have great difficulty achieving a vaginal orgasm because they do not have a thickened G Spot area and/or low PDE5.
Perhaps the G Spot Shot (or G Spot Augmentation) can help these women? I don't think anyone knows the answer to that question fully, but if you've been trying like a mad dog to stimulate the G Spot and nothing much is happening, there may be an anatomical reason for it. Remember that about one third of women report not being able to achieve a vaginal orgasm and perhaps this explains a lot of that phenomenon.
However, the opposite point can also be made: most women have actual erectile tissues in their vagina, Skene's glands, PDE5 and a whole lot of other fun stuff. So guys get busy if you haven't already and you can add a whole new dimension to your woman's sexuality. Most importantly, I can only encourage you to do a little "verification" of the research yourself...
NEWS FLASH: One large study of 1,800 female identical twins (Jan 2010) asked each twin if they had a G-spot. The researchers argued that if one twin had a G-Spot, the other should as well and, therefore, the answers of one twin should match the other. They actually did not find that to be the case and concluded that the G-Spot "doesn't appear to exist".  Back and forth go the arguments, eh? In my opinion, it is a flawed approach to rely on a subjective answer from a woman on this topic: many women struggle to even have an orgasm or know its location for a variety of reasons.
1) Journal of Urology, 174(4):1189-1195, H. O'CONNELL, K. SANJEEVAN, J. HUTSON, "ANATOMY OF THE CLITORIS"
2) American J of Obstetrics and Gynecology, Jan 9 2001, 185(2):359-62, "The G-spot: a modern gynecologic myth"
3) Am J Obstet Gynecol, 2001, 185:359-362
4) Urology, 2002, 60(2):1077-1082
5) J Sexual Med, published online Mar 4, 2008, Jannini, "Measurement of the Thickness of the Urethrovaginal Space in Women with or without Vaginal Orgasm."