Available over the counter as STAMINA Rx and Spontane ES. 

(Today, the Food and Drug Administration (FDA) is warning consumers not to purchase or consume the following products: SIGRA, STAMINA Rx and STAMINA Rx for Women, Y-Y, Spontane ES and Uroprin, manufactured by NVE, Inc., in Newton, N.J. and distributed by Hi-Tech in Norcross, Ga. These products, which are being marketed as dietary supplements, actually contain a prescription drug ingredient that poses possible health risks. The products are being sold over-the-counter and are claiming to increase stamina, confidence and performance.)

In a switch over study  66% preferred Cialis to Viagra, which confirms Danny’s observation as to duration of erection.  

A multicenter, randomized, double-blind, crossover study of patient preference for tadalafil 20 mg or sildenafil citrate 50 mg during initiation of treatment for erectile dysfunction.
Clin Ther. 2003 Nov;25(11):2709-23.
Govier F, Potempa AJ, Kaufman J, Denne J, Kovalenko P, Ahuja S.

Department of Urology, Virginia Mason Medical Center, Seattle, Washington 98111, USA. urofeg@vmmc.org

BACKGROUND: Tadalafil is a phosphodiesterase 5 (PDE5) inhibitor approved in >30 countries for the treatment of erectile dysfunction (ED). It has been shown to improve erectile function compared with placebo in Phase III studies, but clinical experience comparing tadalafil with the PDE5 inhibitor sildenafil citrate is lacking. OBJECTIVE: This study compared patient preference for tadalafil 20 mg or sildenafil 50 mg during initial treatment for ED. It also compared the tolerability of the 2 agents at these doses. METHODS: This randomized, double-blind, fixed-dose, 2-period crossover trial took place at 13 sites in the United States and Germany. Patients were randomized 1:1 to receive 4 weeks of treatment with tadalafil 20 mg or sildenafil 50 mg, followed by the alternative treatment, to be taken as needed up to once daily before sexual activity. RESULTS: The study enrolled 215 men with ED, 109 randomized to the tadalafil-sildenafil sequence and 106 to the sildenafil-tadalafil sequence. Their mean age was 49.8 years; 84.7% were sildenafil naive and 15.3% had undergone a previous inadequate trial of sildenafil. Most patients had moderate ED (60.5%) of >or=1 year's duration (74.9%). Of 190 evaluable patients, 126 (66.3%) preferred to initiate treatment with tadalafil, compared with 64 (33.7%) with sildenafil (P < 0.001). Patients' preference did not differ by age, duration of ED, treatment sequence, or previous sildenafil exposure. Both medications were well tolerated, with no significant differences in the incidence of treatment-emergent adverse events. Headache (11.2% tadalafil, 8.8% sildenafil), dyspepsia [indigestion](6.0% and 4.2%, respectively), nasopharyngitis [inflammation of the nose and pharynx](4.7% and 2.8%), and flushing (2.8% and 4.7%) were the most common adverse events. The rate of ocular disturbances was low: 1 patient experienced intermittent bilateral reduction in visual acuity with tadalafil, and 2 exhibited conjunctival hyperemia or eyelid edema with sildenafil. CONCLUSIONS: Tadalafil 20 mg was preferred to sildenafil 50 mg for the initiation of ED therapy in this study population. Both medications were well tolerated.