Yohimbine is an alkaloid found in two plants, Pausinystalia yohimbe and Rauwolfia serpentina. It has documented properties as both a stimulant and penile-erectile enhancement properties, and reportedly works as an aphrodisiac in some subjects.  Yohimbine, typically extracted from the Yohimbe (Pausinystalia yohimbe) plant, is available over the counter as an aphrodisiac and stimulant fat burner, and is also sometimes used medically as part of a comprehensive erectile dysfunction (ED) treatent. 
Yohimbine has activity at the α2A-adrenergic receptor and is being investigated in both human and animal subjects as treatment for type 2 diabetes who also have polymorphic α2A-adrenergic receptor genes.  It is also useful in tests for simulating anxiety and inducing depressive states (after the stimulant effect subsides) in animal models. [9,10]
According to the NIH and other sources, yohimbine is an effective drug when prescribed for erectile dysfunction. [2,3] Other studies suggest that it is not effective in more severe cases, and have found that the reported increase in libido (as opposed to potency) is anecdotal at best.  Morales et al argue that "conflicting results available may be attributed to differences in drug design, patient selection, and definitions of positive response." 
Administering yohimbine to male rats has been shown to decrease the sexual refractory period, reduce sexual exhaustion, and reduce sexual satiety.  The drug also increased the volume of ejaculated semen in dog subjects, lasting five or more hours post-administration.  According to Adeniyi et al, yohimbine is effective in the treatment of orgasmic dysfunction in men, which is not to be confused with erectile dysfunction or the spectrum of desire disorders. 
According to findings in mice by Dhir et al, yohimbine may have potential in humans for treatment of sexual side effects caused by selective serotonin reuptake inhibitors and for female hyposexual disorder, as well as acting as a potentiation agent for the antidepressant effects of fluoxetine or venlafaxine.  Interestingly, yohimbine has been used experimentally in the treatment of post-traumatic stress disorder to aid recall of traumatic events in a therapeutic setting, possibly by duplicating the effects of stress on the brain and acting as a "trigger."  For this reason, individuals who have experienced post-traumatic stress disorder-like symptoms should probably avoid casual use of yohimbine. 
According to Ostojic, soccer players experienced significant fat loss due to yohimbine.  Yohimbine acts as a fat reduction agent not through a pathway of lipolysis but rather through increased resting energy expenditure: " Yohimbine significantly increased the mean weight loss in patients on a low-energy diet: 3.55 +/- 0.24 kg
(yohimbine) vs. 2.21 +/- 0.37 kg (placebo), P less than 0.005. With yohimbine, a steady level of effort-induced energy expenditure and sympathetic system activity was maintained. " 
Yohimbine is promising as an adjunct treatment for erectile dysfunction; according to Dinsmore, "[t]he alpha-receptor antagonist yohimbine has been found to be effective in some placebo-controlled trials, but its effectiveness is probably inadequate for treatment of most ED." One might thus infer that a dose reduction in more powerful pharmaceutical drugs such as sildenafil or tadalafil might be achieved with the addition of low-dose yohimbine. 
McKay attribute penile erection-enhancing properties of yohimbine to "endothelial L-arginine-nitric oxide activity." 
Pushkar et al found that "the ability of [yohimbine] in a single dose of 5-10 mg to enhance arterial blood inflow to cavernous bodies of the penis was confirmed by dynamic angiopenoscintigraphy and Doppler ultrasonography. [Yohimbine] hydrochloride in a mean daily dose of 15-20 mg proved effective in erectile dysfunction--38 to 84% responders depending on the type of erectile dysfunction. Occasional side effects can be relieved by reducing the drug dose." 
In conclusion, yohimbine is a very versatile plant alkaloid, the potential of which has not yet been fully explored clinically. Like many effective plant alkaloids used traditionally, it offers expected (impotence, fat loss) benefits as well as unexpected potential (enhancement of antidepressants, scientific exploration of the nature of trauma), and like nearly all effective treatments - plant-derived or synthetic - must be taken seriously as it is not without side effects.
Rosengren et al epublished in Science, November 19, 2009
 National Institutes of Health: Medline Plus. "Yohimbe bark extract (Pausinystalia yohimbe Pierre ex Beille Rubiaceae)" http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-yohimbe.html
 National Institutes of Health: National Center for Complementary and Alternative Medicine. "Herbs at a Glance: Yohimbe." http://nccam.nih.gov/health/yohimbe/
 Andersson KE (September 2001). "Pharmacology of penile erection". Pharmacological Reviews 53 (3): 417–50.
Andersson, Karl-Erik; Steers, William D. (1998). "The pharmacological basis of sexual therapeutics". in Morales, Alvaro. Erectile dysfunction: issues in current pharmacotherapy. London: Martin Dunitz. pp. 97–124 .
 Fernández-Guasti A, Rodríguez-Manzo G (July 2003). "Pharmacological and physiological aspects of sexual exhaustion in male rats". Scandinavian Journal of Psychology 44 (3): 257–63.
 Yonezawa A, Yoshizumii M, Ebiko M, Amano T, Kimura Y, Sakurada S (October 2005). "Long-lasting effects of yohimbine on the ejaculatory function in male dogs". Biomedical Research 26 (5): 201–6.
 Adeniyi AA, Brindley GS, Pryor JP, Ralph DJ (May 2007). "Yohimbine in the treatment of orgasmic dysfunction". Asian Journal of Andrology 9 (3): 403–7.
 Dhir A, Kulkarni SK. Effect of addition of yohimbine (alpha-2-receptor antagonist) to the antidepressant activity of fluoxetine or venlafaxine in the mouse forced swim test. Pharmacology. 2007;80(4):239-43. Epub 2007 Jul 6.
 van der Kolk, Bessel A. (1995). "The Treatment of Post Traumatic Stress Disorder". in Hobfoll, Stevan E.; De Vries, Marten W.. Extreme stress and communities: impact and intervention. Boston: Kluwer Academic Publishers. pp. 421–44. ISBN 978-0-7923-3468-2.
 Morgan CA, Grillon C, Southwick SM, et al. (February 1995). "Yohimbine facilitated acoustic startle in combat veterans with post-traumatic stress disorder". Psychopharmacology 117 (4): 466–71.
 Ostojic SM (2006). "Yohimbine: the effects on body composition and exercise performance in soccer players". Research in Sports Medicine 14 (4): 289–99.
 Kucio C, Jonderko K, Piskorska D. Does yohimbine act as a slimming drug? Isr J Med Sci. 1991 Oct;27(10):550-6.
 Dinsmore WW. Available and future treatments for erectile dysfunction. Clin Cornerstone. 2005;7(1):37-45.
 McKay D. Nutrients and botanicals for erectile dysfunction: examining the evidence. Altern Med Rev. 2004 Mar;9(1):4-16.
 Pushkar' DIu, Segal AS, Bagaev AG, Nosovitskiĭ PB. Yohimbine in the treatment of erectile dysfunction (trans. Russian). Urologiia. 2002 Nov-Dec;(6):34-7.