"Good" versus "Evil", Ying and Yang, apparently, is what the Fenphedra diet pill is all about. The "Good" is CART, or Cocaine and Amphetamine Regulated Transcript (hence C.A.R.T), and the "Evil" is Neuropeptide-Y (NPY). Fenphedra claims to have developed the weight loss formula that stimulates CART while inhibiting NPY resulting in weight loss. What? What is CART vs NPY?
Okay, putting Fenphedra aside for a moment, let's discuss what's all this CART vs NPY or Good vs Evil is all about. CART, or Cocaine and Amphetamine Regulated Transcript, is actually a protein which appears to have roles in reward, feeding, and stress . CART is a neuropeptide that has properties of a psychostimulant which induce temporary improvements in mental and/or physical functions such as increased alertness, and higher energy levels. It is believed that CART plays an important role in how the brain regulates hunger. Studies have that when CART peptide was injected into the brain of rats, their food intake reduced by up to 30%. 
On the other hand, there is NPY, or Neuropeptide-Y. NPY also plays an important role in the brain including regulation of energy, memory, and learning. It increases food intake and decreases physical activity.  Needless to say, having excess NPY would lead to increased food intake and hence increased weight gain. Studies on Neuropeptide Y involving rats found that increasing NPY contributes to obesity.  With that said, what Fenphedra claims is that their fat burner stimulates CART and inhibits NPY, leading to successful weight loss - or so they claim.
The Truth About Fenphedra
Fenphedra contains 5 ingredients. Four of the ingredients (Dicaffeine malate, Chocamine, PEA, Synephrine hcl) are the "CART activators", and one of the ingredients (Humulus Lupulus) is the NPY inhibitor - claims Fenphedra. Here's the supplement facts:
Caffeine - Dicaffeine malate is caffeine combined with Malic acid. Dicaffeine malate has similar stimulant effects as caffeine without causing upset stomachs. The stimulant and weight loss effects of caffeine is well documented, and it is one of the most commonly used ingredients in all types of fat burners and diet pills. Without going into much more details about caffeine, I think most would have a decent understanding of what caffeine is and what it does.
Chocamine in Fenphedra is a cocoa extract that is meant to provide the stimulant effects and health benefits of chocolates. Chocamine offers various benefits such as increasing energy, reduce appetite, improve mood and alertness, increased weight loss, and aphrodisiac properties - much like chocolates. Some studies have even found cocoa extract to help inhibit the growth of cancer cells. In a study done by Jourdain et al., it was found that at highest concentrations, cocoa extract induced a complete inhibition of metastatic and nonmetastatic cancer cell lines. They also noted that cocoa extract has an antiproliferative effect on prostate cancer cell growth but not on normal cells. 
Cocoa extract is also known to have mood lifting, and anti-depressant effects . Just like how many people get a mood-lift from eating chocolate, cocoa extract has similar mood enhancing effects. Cocoa extract also has antioxidant properties. Studies have found cocoa extract supplementation has an effect on glucose control, reduce circulating plasma free fatty acid, and also may enhance the antioxidant defense system.  In a Malaysian study, it was found that Cocoa extract significantly lowered serum glucose levels, significantly reduced total cholesterol levels, and improved lipid profiles. 
Phenylethylamine (PEA) helps increase
attention and activity and has been shown to relieve depression
in 60% of depressed patients. Chocolate bars also contain Phenethylamine (PEA).
Synephrine is promoted as a fat loss supplement; however, its effectiveness is widely debated. Synephrine is derived from the fruit of citrus aurantium, which is also known as Bitter Orange, which has been used in Chinese
medicine to treat chest congestion. It is a popular ingredient in many weight loss products and diet pills. Currently, there are conflicting evidence and views on the effectiveness of bitter orange.
Citrus aurantium and synephrine alkaloids in the treatment of overweight and obesity: an update. Haaz S, Fontaine KR, Cutter G, Limdi N, Perumean-Chaney S, Allison DB.
Division of Rheumatology, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Obesity is a major health problem facing the developed and developing world. Efforts by individuals, health professionals, educators, and policy makers to combat the escalating trend of growing obesity prevalence have been multifaceted and mixed in outcome. Various dietary supplements have been marketed to reduce obesity. These products have been suggested to accomplish this by decreasing energy intake and energy absorption, and/or increasing metabolic rate.
Citrus aurantium, which contains several compounds including synephrine alkaloids, has been suggested as a safe alternative to ephedra. This review examines the evidence for safety and efficacy of C. aurantium and synephrine alkaloids as examined in animal studies, clinical weight loss trials, acute physiologic studies and case reports.
The study concludes that while some evidence is promising, larger and more rigorous clinical trials are necessary to draw adequate conclusions regarding the safety and efficacy of C. aurantium and synephrine alkaloids for promoting weight loss. 
In another study conducted at Georgetown University in the US, the researchers cited the fact that Seville orange (Citrus aurantium) extracts are being marketed as a safe alternative to ephedra in herbal weight-loss products, but it may also have the potential to cause adverse health effects. Citrus aurantium contains synephrine, which is structurally similar to ephedrine. It has been found to increase blood pressure in humans and other species, and has the potential to increase cardiovascular events. The researchers went on to say that "There is little evidence that products containing C. aurantium are an effective aid to weight loss."  It is apparent that there are conflicting evidence and views based on various scientific studies on the effectiveness of Synephrine - the two studies we briefly mentioned here are good examples.
Humulus Lupulus is a small genus of flowering plants that grows in the northern hemisphere. The extract of Humulus Lupulus is known to have sedating effects, has sleep enhancing properties, and also has anti-depressant activities. One study by Schiller H et al., have found Humulus Lupulus extract to have sedating properties where it reduced the spontaneous locomotor activity, increased sleep time, and reduced body temperature.  Other studies with rats have also confirmed this with similar findings. Humulus Lupulus was found to have have sleep enhancing and anti-depressant properties.  Some studies have also found it to effectively inhibit proliferation of prostate cancer cells. 
It certainly looks like Fenphedra has plenty of mood enhancing ingredients with anti depressant effects such as PEA, Chocamine (cocoa extract), and humulus lupulus. Perhaps they should have marketed their fat burner as a mood lifter instead. Sarcasm aside, I don't find anything in Fenphedra that warrants the overly high retail price of $179.99, but if you take advantage of their "special offer now", you can buy Fenphedra for $69.99 per bottle. Talk about over pricing. There are plenty of fat burners on the market with similar or better lineup of ingredients that will cost less. Because Fenphedra contains stimulants, there are some potential side effects such as dizziness, jitteriness, upset stomach, increased heart rate, shakiness, and synephrine has also been found to increase blood pressure.
1. Wikipedia - CART http://en.wikipedia.org/wiki/Cocaine-_and_amphetamine-regulated_transcript
2. Science Daily - Cocaine Studies Reveal New Medications For Addiction; How Brain Regulates Hunger