The physiological effects of synthetic cannabinoids include increased heart rate and blood pressure. Psychological effects among users include miild euphoria, relaxation, changes in perception of time and surroundings, and intense sensory experiences. Short-term memory and reflexes become impaired. Hospitals have reported cases of seizures among users who had no prior neurological problems, but there is no good data revealing the likelihood of such occurrences.
The molecules of lab-produced cannabinoids have tail projections similar to that of THC molecules found in marijuana. This allows them to interact with the CB1 receptors found mainly in the brain and which are "normally reserved" for the body's own anandamide, a neurotransmitter that plays a role in pain, pleasure and appetite. The cannabinoids, for the most part were originally produced because they were (and still are) viewed as potential analgesics and as drugs that can offset the side effects of chemotherapy.
The concoction of synthetics found in Black Mamba include JWH-073, JWH-175, JWH-018 and analogues of CP-47,497. Compared to THC, synthetics bind more strongly to the receptors, accounting for the more pronounced physiological effects. When compared to synthetic opiates,
cannabinoids feature far more variation in structure. Even the length of the key binding hydrocarbon tail varies. In JWH-175 and JWH-018, the tail consists of 5 carbons(like THC) instead of the 6 found in CP 47,497 and its analogues(like anandamide), and JWH-073 only has 4. Moreover, there are widely different molecular skeletons attached to the tail, ranging from a heterocyclic system in the JWH series to the phenolic groups in the CP analogues. This is pure speculation on my part, but perhaps there are far more cannabinoid receptors involved than the ones identified so far. In smell, for instance, where varying structure among similar molecules initially caused much confusion among scientists trying to understand the mechanism, it turned out that the same odiferous molecule has to interact with several different nasal receptors for the signal to get to the brain.
If one examines the chemical analyses of Spice or Black Mamba, hoping for consistency in composition, one will be disappointed. People producing these drugs seem to toss in a variety of cannabinoids, similar to the way that many psychedellic drugs sold in the late 1970's with the "acid" or "mescaline" label actually contained amphetamine derivatives. Most of the synthetic cannabinoids were legal until recently. Beginning in March 2011, US Drug Enforcement Administration placed 5 synthetic cannabinoids into "schedule 1" for a 12-month period. This means that they are now in the same class as other "hard drugs", so possession is illegal. Prior to that, states had taken upon themselves to ban them.
Illegalization, of course, causes a spinoff of problems. Research, education, and centralized quality control are a better alternative.
Sources:
1. Synthetic Cannabinoids in Oral Fluid
http://www.ingentaconnect.com/content/pres/jat/2011/00000035/00000007/art00005
2. The Secret “Spice”: An Undetectable Toxic Cause of Seizure
http://nho.sagepub.com/content/1/4/182.extract
3. Spice, K2 and the Problem of Synthetic Cannabinoids
https://www.ncjrs.gov/App/Publications/abstract.aspx?ID=255578
4. Wikipedia:
Cannabinoids
JWH-018 and similar articles