As I write this, I am actually drinking hibiscus tea. There is no doubt that it is Mother Nature's gift to every man on the planet (especially if you are over about 40). It's quite bitter, but then so is beer, eh? My experience is that you get used to the bitterness and actually enjoy the taste after a few days.
So what's all the hibiscus hype? Well, it's not hype and let me prove it to you with a number studies from the research that should get your attention. Remember that erections are all about your arteries - nitric oxide in your arteries and nice low blood pressure levels (which indicates higher blood flow):
1. Increased eNOS Activity and Nitric Oxide. We get most of our arterial nitric oxide from the endothelium and this is governed by the eNOS enzyme. It turns out that the polyphenols in hibiscus tea activate this enzyme and cause your endothelium to produce more nitric oxide. 
Now you may object and say, "Well, this was in the lab and not in actual human subjects." That is true. However, one of the big tests for any possible NO-increasing compounds is whether or not it can produce big drops in blood pressure. As you'll see below, this has been demonstrated in at least five studies, three of which I have documented below.
Also, I do not need to point out that boosting arterial nitric oxide, which is what we are talking about here, will generally help significantly with mild to moderate erectile dysfunction. No one can promise you it will solve it, but you have little to lose, eh? Hibiscus tea is very easy to find and prepare, something I discuss here: How to Make Hibiscus Tea.
2. Endothelial Function. Remember this formula:
Erectile function = Endothelial function
The endothelium is the thin layer of cells on the inside of your arteries that are responsible for controlling their expansion and contraction and the nitric oxide that governs the process. Researchers refer to this ability to relax the arteries as "endothelial function," and, generally speaking, endothelial function governs how well and how fast your erections are. (There are exceptions, of course, as low dopamine, venous leakage and other systems can negatively impact erections as well.) In any event, you can probably guess where I am headed: hibiscus tea has been shown to significantly help with endothelial function:
"Diuresis and inhibition of the angiotensin I-converting enzyme were found to be less important mechanisms than those related to the antioxidant, anti-inflammatory, and endothelium-dependent effects to explain the beneficial actions. Notably, polyphenols induced a favorable endothelial response that should be considered in the management of metabolic cardiovascular risks." 
Notice that these researchers boldly told physicians to consider using hibiscus in their practices. Amen! Oh course, how many physicians actually consider natural solutions to chronic disease, even though 99% of the time, chronic disease results from unnatural lifestyles?
Again, an improvement in endothelial function will help the solid majority of men with mild to moderate erectile dysfunction. This is especially true if coupled with a high nitric oxide-based lifestyle, something I explain in great detail my my book: The Peak Erectile Strength Diet.
3. Hypertensive Type II (Adult Onset) Diabetics. This study did a black tea versus hibiscus tea study. (Black tea is known for also improving endothelial function, although this study found the opposite.) The results were very impressive for hibiscus tea:
"The mean of systolic BP (SBP) in the ST [hibiscus tea] group decreased from 134.4 11.8 mm Hg at the beginning of the study to 112.7 5.7 mm Hg after 1 month, whereas this measure changed from 118.6 14.9 to 127.3 8.7 mm Hg in the BT group during the same period." 
A 20+ point drop in systolic blood pressure should get everyone's attention. Of course, this means that the participants arteries relaxed or dilated and blood flow increased.
Admittedly, the number of patients was small (60) and another study found that black tea decreased blood flow in a dose dependent manner  instead of restricting it.
4. Moderate Essential Hypertension. Patients with essential are often the toughest to treat, because they have no known cause. In a small study, hibiscus provided solid results in just this situation:
"Statistical findings showed an 11.2% lowering of the systolic blood pressure and a 10.7% decrease of diastolic pressure in the experimental group 12 days after beginning the treatment, as compared with the first day. The difference between the systolic blood pressures of the two groups was significant, as was the difference of the diastolic pressures of the two groups. Three days after stopping the treatment, systolic blood pressure was elevated by 7.9%, and diastolic pressure was elevated by 5.6% in the experimental and control groups." 
5. Prehypertion and Mild Hypertension. Again, this was a fairly small study (65 participants) but provided solid results:
"A standardized method was used to measure BP at baseline and weekly intervals. At 6 wk, hibiscus tea lowered systolic BP (SBP) compared with placebo (-7.2 11.4 vs. -1.3 10.0 mm Hg). Diastolic BP was also lower, although this change did not differ from placebo (-3.1 7.0 vs. -0.5 7.5 mm Hg)." 
This group is particularly relevant in my opinion, because we clearly have quite a few prediabetic men on the Peak Testosterone Forum.
CAUTION: Do not drink too much hibiscus tea. One of the men on the forum said that Dr. Greger advises less than a quart to make sure you don't get too much manganese. The bottom line is that more is not always better. Also, if you have low blood pressure or are on blood pressure medications, please discuss with your doctor first.
6. Protection Against Arterial Plaque. One thing that can lower your nitric oxide and endothelial function over time is a buildup in plaque in your arteries. Why? Because plaque represents actual damage and "scarring" of the lining of your arteries. Keep in mind that blood supplied to your penis is done so via the pudendal artery and you have actual penile arteries as well. Things simply are not going to work right in the bedroom if you have significant atherosclerosis. I have a page that covers the association between the buildup of plaque and the loss of erectile strength: Evidence That a Loss in Morning Erections Is Due to a Buildup of Arterial Plaque.
SOUR GRAPES: Meta-Analysis. Oy vay is all I can say for what these researchers concluded:
"Results Four trials, with a total of 390 patients, met our inclusion criteria. Two studies compared Hibiscus sabdariffa to black tea; one study compared it to captopril and one to lisinopril. The studies found that Hibiscus had greater blood pressure reduction than tea but less than the ACE-inhibitors. However, all studies, except one, were short term and of poor quality with a Jadad scoring of <3 and did not meet international standards. Conclusion The four randomized controlled studies identified in this review do not provide reliable evidence to support recommending Hibiscus sabdariffa for the treatment of primary hypertension in adults." 
The authors above know that no one except large pharmaceutical companies can afford massive, large scale trials. So, sure, it's obvious that hibiscus tea has not been held up to FDA-style scrutiny, and it never will. But it's been a solid performer in multiple studies on multiple subpopulations, so why not just try it? It's a common drink in many countries, and so it's safety profile should be considered stellar compared to virtually any phamraceutical.
CONCLUSION: Hibiscus tea provides solid results for increasing blood flow and lowering blood pressure and should be considered by men with erectile dysfunction as a potential significant help. It should also be consumed in my opinion regularly to help prevent atherosclerosis.
1) Journal of Human Hypertension, 2009, 23, 48 54, "The effects of sour tea (Hibiscus sabdariffa) on hypertension in patients with type II diabetes"
2) Journal of Hypertension, Apr 2009, 27(4):774 781, "Black tea consumption dose-dependently improves flow-mediated dilation in healthy males"
3) J Ethnopharmacol, 1999 Jun, 65(3):231-6, "The effect of sour tea (Hibiscus sabdariffa) on essential hypertension"
4) J. Nutr, Feb 2010, 140(2):298-303, "Hibiscus Sabdariffa L. Tea (Tisane) Lowers Blood Pressure in Prehypertensive and Mildly Hypertensive Adults"
5) Phytomedicine, Feb 2010, 17(2):83 86, "The effectiveness of Hibiscus sabdariffa in the treatment of hypertension: A systematic review"
7) Mol. Nutr. Food Res, 2014, 58:1374 1378, "Hibiscus sabdariffa extract lowers blood pressure and improves endothelial function", http://onlinelibrary.wiley.com/doi/10.1002/mnfr.201300774/pdf