I will be honest. I never heard of it, before you asked.
I have taken a quick run down of the wikipedia page on it and reflected on the theory for an hour or so. My opinion is thus no better supported than the quality of the wiki article.
That caveat presented, I would say, I find the theory pretty @#$%& brilliant. As far as I can see, it is a natural consequence of Darwinism. Darwinian evolution demands, that every time there is a instability in a given biological system, there is a niche for organism to exploit. With evolution, that will happen eventually, and that species will thrive, opening up other instabilities that other creatures can exploit.
I see that one of the major critics is Richard Dawkins, author of The Selfish Gene, which is brilliant book. It is ridiculous that he is a critic of the theory, as he probably the biggest Darwinian on the planet, but there you go. Several other Darwinians have joined him in this critique, and it seems to me, that they fail to realize (or maybe the material that describes the theory fails to properly explain), that the reason for this mechanism, is not inside the living organisms, but it is a consequence of the laws that govern those organisms.
I also see that the Gaia theory has been criticized for being teleological, which means it runs towards an end purpose... well hello?!? So does the theory of the Big Bang (the universe will either collapse on itself or expand until it disperses its energy so much that it will freeze over. We don't know which one, but regardless of that, it is inevitable). Darwinism is also teleological if you think about it. Teleological arguments are also used in physics and chemistry. In any case, this is used to link the theory to new age hippie circles, thus trying to discredit it as a spiritual movement rather than a theory by association, because teleological traditionally have been linked to God's will. I find this an unsavoury way to attempt to discredit a theory.
I do see a weakness though. According to the theory it is the interaction of the biomass, understood as the biosphere, through the atmosphere, oceans, and soil that forms the entity and hence the self-regulating cybernetic system. In this biomass, humans, and hence human activity, are also a part. Volcanos, solar activity and meteors are not. The theory itself relies on the process of evolution (that is life!) to create the equilibrium.
Humans are the only creature on the planet, that has harnessed the ability to create systemic instability, and then exploit it ourselves. The entire human endeavour since we became sentient, has been to work on speeding up and further develop this capacity. At first it wasn't much, like fire, agriculture, animal husbandry and so on. But now we are at a point, where we can change the entire face of the planet in less than a generation, should we wish to do so. With an atomic holocaust, we can do it in less than an hour!
In short: We can radically change the fundamental conditions for all other forms of life, faster than any evolutionary process can ever handle. Thus, even though we are a part of evolution, in many more ways than we can understand, we are also somehow above it, because we act as a much stronger force on life, because we can act much faster than evolution can respond. That is what has set us at odds with the rest of the biosphere. But the theory holds biosphere as biomass, which we are a part of. This is most problematic.
To further clarify: We know how fast evolution can work. Some of the best examples are large animals, that become trapped on small islands, with no natural predators. Let us have a group of horses stranded on an island that is 100 square kilometres. With no natural predators, they will breed, and their numbers will grow unchecked. The main evolutionary pressure will be as always, the ability to survive and the ability to breed. In this case, that will be determined by the very limited amount of plant matter to be eaten. As numbers increase, the share of food available to each horse dwindles. Thus ability to secure and utilize food will be paramount both for survival and replication.
Situations like this have happened many times during the course of natural history. The successful evolutionary response is usually miniaturisation. This might seem odd, as a smaller horse should have LESS chances to reproduce. That might be true, but in a famine situation, as we have generated, it will be best suited to survive, and hence replicate. Large horses are not strong when they starve, and thus do not reproduce, as they will be beaten be smaller horses that are well-fed. Thus the horses shrink in size with each new generation.
This is perhaps one of the most extreme forms of evolutionary pressure that you can generate that doesn't wipe out the test populace before it can adapt. As a consequence, evolution is working in overdrive. But even in these conditions, it still takes about 6000 years for the horses to adapt (a scary consequence of this, is that our emotional response system, thinking capacity and instincts are most likely wired up to handle the life of the caveman!). Given this, all large forms of life are screwed, when man is around. With size usually comes a slow reproduction cycle, and that is the real killer, as evolution does not work in units of years, but in units of generations.
This being the fact, it seems the only forms of life that are thriving with our presence, is stuff like viruses, bacteria, amoeba, algae and stuff like that... maybe even banana flies can keep up. But as we all know, birds and frogs cannot.
Thus I cannot see, we as humans, who are part of the biomass, framed in the term "biosphere", are following the theory as it is described. We are not trying to establish "optimal physical and chemical environment for life on this planet". It is my honest opinion, that the Gaia theory should be able to account for that, and while I can see human activity has been discussed extensively, I cannot see this particular point has been addressed. If this critique can be developed further, it probably has the capacity to snowball into a mortal wound for the theory.
Taking the opposite stance, if we assume, that the theory is correct, and that there is a proper explanation for my critique, then it would follow, that the planet as a cybernetic system, would work very hard to exterminate us. This is also discussed in "The Revenge of Gaia". I have to say, despite my own critique seems intellectually strong to me, my gut feeling is going with "The Revenge of Gaia".
As Agent Smith puts it in "The Matrix":
"I'd like to share a revelation I've had during my time here. It came to me when I tried to classify your species. I realized that you're not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed. The only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet. You are a plague, and we are the cure."
It's only after we've lost everything, that we are free to do anything.