Here's something the pharmaceutical industry will not tell you about their precious PDE5 Inhibitors: many men develop "Viagra Resistance". Several studies have shown that you can because resistant to the erectile dysfunction drugs Viagra and Cialis over time.
What does this mean? It means that such men will find that they have to up their dosage over time. This is ugly, because these drugs are rife with side effects. Having to steadily increase one's dosage will simply increase the risk for issues. (Read my link on PDE5 Inhibor Side Effects and The Dangers of Cialis for more details.)
One 2006 study mentioned that not only do these drugs often have "decreased efficacy" but also often end in "tachyphylaxis," the medical term for a "rapid decrease in responsiveness" to a drug.  The authors note that "treatment failure may cause considerable distress". This has to be one of the scientific understatements of the year.
Just how big of a problem is this? Well, no large studies have been done to date, but one smaller study showed clearly this is a serious issue. Researchers looked at men with erectile problems that had taken Viagra successfully and continued use over an extended period of time. Of these men, fully 37% had to increase their dosage by 50 mg! In a second cohort 50% of the Viagra users discontinued due to the decreased effectiveness of the drug. 
CAUTION: Do not quit any medication without consulting your doctor. Viagra and Cialis can lower blood pressure slig
This also creates a path to dependency as well. Imagine this all-too-common scenario: you start on Viagra or Cialis and notice some slight side effects, maybe mild ringing in the ears, visual issues or stomach upset. You are so happy to get your erections back that you ignore any negative aspects. However, as the months roll on, the nagging doubts increase: "What is this ringing in my ears doing to my hearing long term?" Or maybe the stomach upset or headache just gets worse over time to the pont where "the cure is worse than the disease."
In the above study  the average time until one had to increase dosage was 11 months. Therefore, most men finding themself in this situation are going to be relatively long term users that are going to find quitting difficult indeed. In fact, this can leave a guy with a major dilemma: if he quits these PDE5 Inhibitors cold turkey, he will likely find that his erectile dysfunction is worse than ever because he has developed an actual dependency. His erectile situation is now worse than ever before, because now he has to overcome both his original erectile dysfunction AND his Viagra Resistance.
What is the solution? Again, consult with your doctor, but read my link on Eliminating Cialis and Viagra Dependency for practical suggestions to discuss with him or her.
1) Curr Vasc Pharmacol, 2006 Apr, 4(2):89-93, "The management of phosphodiesterase-5 (PDE5) inhibitor failure"
2) The Journal of Urology, Sep 2001, 166(3):927-931, "LONG-TERM EFFICACY OF SILDENAFIL AND TACHYPHYLAXIS EFFECT"