If you frequent the board on the Peak Testosterone Forum, you will find that there are a big percentage of men that are on an aromatase inhibitor, especially Arimidex (anastrozole). In fact, you'll find that, even though I am attempting to run a natural men's health web site, I myself am on Arimidex (albeit at a very low dosage). What gives? Is this sheer hypocrisy or madness? Or is there a non-disturbing, rational explanation for this?
Before we go into this, let's discuss just what an aromatase inhibitor does. Aromatase is the enzyme that converts testosterone into estradiol is us men. As it turns out, both aging and the weight gain that accompanies aging leads to greater conversion of our testosterone into estradiol, the "bad estrogen", that is responsible for so many well-known issues in men, including (often) prostate issues, gynocomastia and erectile/libido/hormonal problems. Aromatase is in fat cells and as our fat cells expand from a sedentary lifestyle and/or overeating, we men can easily end up with too much aromatase.
NOTE: Aromatase is also involved in the creation of estrone via conversion from androstendione.
This is where the aromatase inhibitors come in. They work by binding to aromatase, thus taking it "out of commission". So why would so many men be on AI's (aromatase inhibitors)? First of all, a few men on the Forum have tried aromatase inhibitors for medical reasons. For example, estradiol can play a major role in gynocomastia. An aromatase inhibitor such as Arimidex (anastrozole) - occasionally letrozole is used - is used quite commonly to prevent or reverse gyno. (Tomoxifen, which is a SERM, is also used and actually goes after prolactin and probably estrogen receptors in your chest/breast tissue.)
These usages of AI's are relatively uncommon. However, it is Arimidex's usage in HRT (testosterone therapy) that has now become increasingly common. From what I have seen, if a man goes over a total testosterone of about 600 ng/dl, he will very likely need an aromatase inhibitor. In fact, one of the biggest reasons that I see for issues with HRT is when doctors do not treat the elevated estradiol levels that can occur boosting a man's testosterone. The symptoms are very similar to those for low testosterone: mood crashes, loss of libido, erectile dysfunction and so on.
That said, many doctors do know about the importance of managing estradiol with testosterone replacement and will give their patients Arimidex. The dosage is usually very low - .5 to 1.0 mg/ week split it into two - compared to that given to prostate or breat cancer patients. Many men are now going to HRT clinics as well and these almost universally prescribe Arimidex, because testosterone levels are typically pushed into the 800-1200 range (peak). The purpose of the Arimidex is to get men into the 20-30 pg/ml range. And there have been literally dozens of men on Arimidex on the forum following this or a similar protocol.
It should be pointed out though that men can easily get over high estradiol levels from the following situations as well:
1. Being overweight.
2. Being on Clomid to boost testosterone (and preserve testicular function and fertility) or get off of HRT.
3. Being on HCG Monotherapy. (See my page on HCG Monotherapy for more information.)
So, if you put all these cases together, many more men may be using an aromatase inhibitor than you might think, especially as testosterone therapies, both standard and alternative, have grown and expanded in popularity over the last decade. And almost all of them are using Arimidex.
Why, then, the Arimidex obsession? Well, first of all, it is now quite reasonably priced after going generic a couple of years ago. And it has broad application and relatively few side effects if a) used at a lower dosage and b) plasma estradiol levels are regularly monitored and managed.
NOTE: Arimidex is very powerful and higher dosages (1 mg/day) will lower "estradiol by approximately 70% within 24 hours and by approximately 80% after 14 days of daily dosing."  Other studies have shown that you can get even greater reduction in estradiol levels however. 
However, there are definitely other aromatase inhibitor options out there and a couple of them may be more applicable, i.e. discuss with your doctor if appropriate:
1. Letrozole (Femara). One study (in women) showed that Arimidex could achieve almost total suppression of estradiol levels but was still detectable. However, letrozole was even more powerful and could achieve total suppression of estradiol to where it could not even be detected!  Now you have to be careful in going too low with estradiol as it can actually be dangerous and lead to osteoporosis, mood and erectile issues, etc., something I document in my link on "Do Men Need Estrogen?".
However, in some cases, that slight extra horsepower from letrozole can help with gynocomastia, at least according to the "common knowledge" on the steroid forums. The general feedback is that anastrozole can prevent gyno usually, but letrozole can actually reverse it (in some cases). Discuss with your physician of course as letrozole has a repulation for more side effects.
2. Suicide Inhibitors (such as Aromasin). These "type I" type of aromaste inhibitors do their work using a little different technique: they actually bind to the aromatase enzyme and permanently and irreversibly take it out of commission. This may seem really ugly, but the body rebuilds those enzymes after a few weeks usually. These type of inhibitors are popular in the steroid community and men that are doing HRT on their own (which I don't advise). However, I have not seen many HRT clinics, urologists, endos or PCPs. using them.
One study on young men showed that 25 and 50 mg dosages both reduced plasma estradiol levels by about a third in 14 days, which is not a bad reduction.  I cover these types of aromatase inhibitors in more detail in my link on Suicide Inhibitors.
SIDE EFFECTS: Side effects are minimal on these drugs assuming that estradiol levels are kept in a safe range. Most of the nasty side effects come from men and women who have greatly suppressed their estradiol due to cancer treatment. However, it should be pointed out that Arimidex, for example, affects liver enzymes and may also raise inflammatory cytokines. I hope to do a page on this soon.
2) Journal of Clinical Oncology, Feb 1 2002, 20(3):751-757, "Influence of Letrozole and Anastrozole on Total Body Aromatization and Plasma Estrogen Levels in Postmenopausal Breast Cancer Patients Evaluated in a Randomized, Cross-Over Study"
3) The Journal of Clinical Endocrinology & Metabolism, Dec 1 2003 88(12):5951-5956, "Pharmacokinetics and Dose Finding of a Potent Aromatase Inhibitor, Aromasin (Exemestane), in Young Males"