CLA in the past has been viewed with skepticism by many because of the variety of studies, some showing that it has no or negative effects in the body, while others showing it has muscle building and fat building properties. There are many different isomers of CLA, and as such, there is no single version of CLA that has proven to be positive or negative overall, and there has been much debate to this end. CLA is also a trans fat which the USDA has reported that it may be a potential risk factor for cardiovascular and inflammatory diseases.  Shedding a little more light on the differences of the isomers of CLA is an article by Tom Rayhawk for M&M Magazine, entitled "SREBP-1 Proteins, part 2":
CLA or conjugated linoleic acid has received considerable attention in both the medicinal and supplementation industry. Initially, there did seem to be support for its use as a ‘repartitioning’ agent, as CLA improved feed efficiency in mammals, by reducing fat storage pathways and enhancing beta-oxidation through an increase in lipolytic proteins. Their effects, however, seem to be isomer specific. There are two isomers of CLA – cis-9, trans-11 (abbreviated c9,t-11) and trans-10,cis-12 (abbreviated t10,c-12). In essence the ‘good’ characteristics of CLA seem to be attributed to the c9, t11 isomer. In fact, the two seem to have opposing actions... the implications of CLA use depend on the type of isomer used.There is considerably more information on CLA in Mr. Rayhawk's article starting at the bottom of page one and beginning on page two, from what is quoted above, so I highly recommend checking out the full article for the scoop.
So, as you can see, there is much controversy and confusion over the whole issue of CLA supplementation. However, in this most recent study, which is actual a review of other studies, has demonstrated that supplementation with CLA lost an average of 0.2lbs of fat per week more than others who were not using it. It also showed that these effects started to diminish after around six months of continuous usage. The dosage used in the studies was 3.2gm per day. Since most dietary supplements promoting fat loss benefits with CLA contain only 1gm or less of the compound(s), this may be the reason why many failed to see any benefits when using it in a diet and exercise program. For some time, CLA was somewhat of a "fad" supplement that was advertised as being able to increase lean muscle mass, while at the same time decreasing adipose (fat) storage made it high in demand and quite expensive. The general recommended dosage by most who had researched the CLA stated that in needed to be taken in ranges of 5-10gm per day. Obviously, this was not cost effective for most people and with the minimal fat loss that occurred while taking it caused many people to shy away from it entirely. In fact, in a previous study done in 2004 by the same organization, AJCN, using an animal model showed that "the effect of CLA on body fat is considerably less than that anticipated from mice studies and that CLA has no major effect on plasma lipids." 
In 2001, Dr. Lonnie Lowery, a member of the board of directors for the American Society of Exercise Physiologists, attended a conference on CLA supplementation and reported back with his findings in an article, "CLA: Poison or Gift?" for T-Nation. Dr. Lowery reported that the conclusions made were that some of the isomers of CLA were only responsible for muscle increase and no fat loss at all, seemingly contradicting more recent evidence.
First up, Dr. Michael Pariza, a CLA patent holder and "king" of the field, explained the "isomer paradox." What did he mean? For starters, he showed data regarding how the 9,11 isomer of CLA, not the 10,12 version, appears responsible for improving muscle growth. This is interesting because the body can make a bit of 9,11 CLA on its own. Perhaps it is, in fact, a necessary "natural" factor in resistance-training hypertrophy. Regarding body fat, the opposite appears true: only the 10,12 isomer affects fat metabolism. Since the human body can’t synthesize trans-10, cis-12 CLA, its intake from dietary sources (e.g. beef, dairy) appears especially important. He then went on to show that 10,12 CLA has NEVER been shown to have lipolytic (fat burning) effects.These claims are quoted in the article as being from a single source - the conference that was being attended by Dr. Lowery himself. However, the article itself contains much more information on the subject, and a jumble of more confusing and contradicting claims. The final conclusion?
So what’s the final synopsis on CLA after my European escapade? First, CLA certainly has promise for bodybuilders or powerlifters during a bulking phase [emphasis added] (say, October through February). Holiday season festivities and the associated dietary binges should provide plenty of help during this "mass cycle," eh?
Second, the advent of different isomer-specific products may usher in a more "targeted" approach to CLA supplementation. Third, the necessary dose for bodybuilders and powerlifters may far exceed the cancer-fighting and heart disease-combating doses. This is not an excuse to mega-dose, however. CLA interacts with dozens of genes, with, as yet, unknown consequences. And finally, unusual dietary fats other than CLA, like CLNA and others, may make interesting additions to physique athletes’ arsenals one day.
Because research often moves so quickly it can be hard to keep up with, it seems that these assertions from this 6 year old article might be a bit dated. Still, the scientific and bodybuilding world is still divided as to whether or not CLA may build muscle, cause fat loss, do both, or maybe even neither. However, this new study seems to indicate that there is at least cause to believe that CLA can probably cause a small amount of fat loss when used at the proper dosage. That, along with the fact that it's effects seem to taper out after around six months probably help negate some of the potentially negative long term side effects of CLA supplementation.
So, what's my verdict? It appears that currently the cheapest way to use CLA to the full advantage of someone trying to do a lean bulk or lose fat would be to pick up a few bottles of PBL Clarinol CLA Plus, coming in at only $23 for 8oz and a serving size of 3.5gm per 1/2 tablespoon, which would be a full 30 day supply as recommended by the study. It looks to be the best price for the right amount, the only thing that could potentially be a setback is the "orange creamsicle" flavor. But, for anyone who's ever downed pure olive, flax, or even fish oil, that might be a treat. Be aware that CLA appears to deplete levels of DHA (Docosahexaenoic acid) , an omega-3 fatty acid found primarily in fish, so you will probably want to take a fish oil supplement along with it. Whether or not CLA stacks well with other supplements to promote fat loss has not been explored much in a clinical setting, with the exception of chromium, which showed no added benefit.  Looking at animal studies involving high doses of CLA, it also seems to block out other essential fatty acids (EFAs) and their enzyme pathways , so products that include other EFAs, such as VPX's Thinfat or SAN Lipidex could help in preventing deficiencies. For those obviously seeking much more quick and substantial fat loss gains would probably do well to stack CLA with a standard fat burner product, such as German American Technologies JetFuel.
Reuters Health Information (2007-05-31): Fatty acid supplement may aid body fat loss
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