ACE Inhibitors are a widely prescribed class of medications for hypertension (high blood pressure). They have been around so long that doctors rarely explain the side effects, some of which are surprsingly serious and/or common as I will discuss below. (Insurance companies love this class of medication, because, generally, there are cheap.)
If you read about how an ACE Inhibitor works, it is VERY complex and involves multiple systems in the body. Basically, the kidneys release a protein called renin. Renin acts like a filter and plays a role in angiotensen I. The ACE (angiothensin converting) enzyme converts some of angiotensin I to angiotensis II and the latter increases blood pressure and constricts vessels, etc. So someone got the idea of slowing down or inhibiting the ACE enzyme in order to lower blood pressure. Seems easy enough, eh? However, as you will see below, disturbing this extremely complex set of interrelated systems is fraught with risk, some of which could affect your bedroom performance in one way or the other.
There are many research-backed ways to attack hypertension and I outline some of them in my link on 30+ Natural Ways to Lower Blood Pressure. Using natural methods can in many cases lower your need for antihypertensive medications or eliminated it altogether. In fact, I discuss below some natural ACE inhibitors that can be of asistance. (High blood pressure is very serious, so do not do anything without discussing with your physician first.)
Are ACE Inhibitors popular? Yes, because they are cheap and synergistically combined with other antihypertensives for additive effects in some cases. Look at the sheer number of ACE Inhibitors out on the market as well: Altace, Lotensin, Accupril, Capoten, Prinivil, Zestril, Univasc, Vasotec, Monopril and Mavik.
CAUTION: Not everything about ACE Inhibitors is bad. They have some extremely beneficial properties and one must weigh the pros and cons with your doctor. Do NOT quit any medication without first consulting your physician.
That said, "eyes wide open" as I always say. As a patient, you have the right to know how a medication works, what it will do to your body and potential side effects that could be an issue for you down the road. First of all, let's start with a couple of the not-so-well-known benefits of going with synthetic ACE Inhibitors:
1. Improved Sexual Function (in SOME cases). ACE Inhibitors have a spotty record when it comes to bedroom performance for reasons we will discuss below. However, ACE Inhibitors do generally improve endothelial function and blood flow at least. In some men, this helps their sex lives. For example, one study looked at the result of a variety of standard antihypertensive medication, including an ACE Inhibitor (captopril), a calcium channel blocker and a beta blocker.  Which drug was the only one that did not cause sexual dysfunction? You got it: the ACE Inhibitor. Well, actually this study did not say there was an average improvement, but, anecdotally, some men report a benefit.
Most importantly, notice what the study did NOT say: it did not say that erectile strength was improved? Doesn't that strike you as odd? Most of the Supplements that Improve Erectile Function improve bedroom performance and have done well in studies on erectile dysfunction studies. Why wouldn't a blockbuster class of blood pressure lowering drugs be almost as good as Viagra and Cialis? We will discuss some of the reasons for this below.
2. Decrease Inflammation. One of the Holy Grails in the health world is lowering inflammation, especially markers such as C-Reactive Protein and TNF alpha. Furthermore, decreasing inflammation as a principle is usually excellent for erections. One study found that one of the ACE Inhibitors greatly increased blood flow by lowering the above inflammatory cyokines (TNF alpha and CRP).  (CAUTION: One of the ACE Inhibitors in the study did not do much of anything however.)
And an example of the benefits of this are that some ACE Inhibitors (but not all) have been found to significantly lower the risk of developing dementia, a condition that is now thought to be substantially caused by elevated inflammation.
So are ACE Inhibitors then nothing but Goodness and Light? Again, there are a few studies that show improvements in function for men with hypertension.  However, as mentioned above, this is spotty and certainly not what one would expect from something that can so dramatically increase blood flow.
So now let's look at some of the many very common side effects associated with pharmaceutical ACE Inhibitors:
1. Lowered DHEA. DHEA is critical to cortisol control and libido. ACE Inhibitors are known to often lower DHEA levels and this is likely one of the big reasons that ACE Inhibitors do not score as well as they should in the erectile strength department. Yes, Erectile Dysfunction is a Risk for High Blood Pressure , but an erection is complex and lowering DHEA is likely to be a net negative. The evidence is surprisingly strong that DHEA deficiences, for example, can play a role in depression, adrenal insufficiency and possibly chronic fatigue syndrome.  None of these is going to help your sex life!
2. Persistent Cough. This one will seem just a tad bizarre. Imagine a medication where a VERY common side effect is that you will develop a chronic, nagging cough. Welcome to the world of ACE Inhibitors. Approximately 5 to 25% of patients will develop this kind of cough and it can take a couple of weeks for the cough to stop even after the medicaiton is halted. 
3. Erectile Dysfunction. Sexual dysfunction, although not nearly as prevalent as the cough side effect, is widely reported nonetheless. What is worse is that I have never seen a good explanation as to why. This is a good example that they simply do not know or care to know how these pharmaceuticals work.
4. Gynocomastia. Here's one your doctor is unlikely to mention: gynocomastia is a well-known and fairly side effect of the ACE Inhibitors. Again, I have never seen a plausible explanation as to why, showing these drugs are poorly understood.
ACE inhibitors lower blood pressure by preventing the breakdown of bradykinin, a molecule that causes blood vessels to widen. The swelling is believed to be caused by too much bradykinin, which makes blood vessels widen until they leak, letting fluid seep into tissues.
5. Angioderma. ACE inhibitors lower blood pressure by preventing the breakdown of bradykinin, a molecule that causes blood vessels to widen. The swelling is believed to be caused by too much bradykinin, which makes blood vessels widen until they leak, letting fluid seep into tissues. A not-too-uncommon scenario is for an ACE Inhibitor user to experience swelling of the face and neck - sometimes leading to hospitalization or death! - from this type of tissue enlargement. 
One question that may cross your mind: "Are there any natural ACE Inhibitors?" The answer is yes and no. There are not ACE Inhibitors with necessarily the same power as one of the pharmaceuticals. That said, it is curious that the medical community has ignored the natural alternatives that seem to be virtually devoid of side effects:
NOTE: Do not take these if you are on medications without first consulting your doctor. Pomegranate juice is so powerful, for example, that it can alter absorption rates of certain pharmaceuticals.
1. Pomegranate Juice. Pomegrantate Juice has a natural ACE Inhibitor (among other powerful phytochemicals) that can lower blood pressure. I call pomegranate juice my Best Bedroom Buddy, because it also increases nitric oxide and improves lipid profiles and on and on. For more information, read my link on The Many Benefits of Pomegranate Juice.
2. Hawthorne Berry. This fruit is an ACE Inhibitor and, remarkably, its' also a Calcium Channel Blocker and nitric oxide booster.  You can buy it by itself or as NEO 40 (Neogenis Labs) , which has additional ingredients in it including L-Citrulline. You can read more about in my links about The Good and Bad of L-Citrlline and An Interview with Nathan Bryan.
1) Journal of Hypertension. Supplement : Official Journal of the International Society of Hypertension, 1988, 6(4):S649-51], "Effects of first-line antihypertensive agents on sexual function and sex hormones"
2) Eur J Heart Fail (2006) 8 (5): 451-459, "Correlation of flow mediated dilation with inflammatory markers in patients with impaired cardiac function. Beneficial effects of inhibition of ACE"
4) Clinical Cardiology, Jan 2003, 26(1):25-30, "Prevalence and risk factors for erectile dysfunction in men with diabetes, hypertension, or both diseases: A community survey among 1,412 Israeli men"