I have firsthand experience and knowledge with reishi mushroom extract. You can study reishi articles yourself from Pubmed.com for example. Red reishi (Ganoderma Lucidum) is very intresting and potential nutritional supplement for many different things. It is a tree mushroom (grows on dead trees) and probably transforms some of trees superp ingredients to human edible form. Same kind of mushroom is Chaga (Inonotus Obliquus). Reishi has very high or highest meaning in TCM (traditional chinese medicine). TCM considers reishi as the king mushroom of immortality. Wild reishi grows usually on stump of a leafy tree. I have taken reishi extract supplements and made my own chaga extracts. These extracts are very safe but shouldn't be unerestimated. There are at least 7 herbal components in reishi such as germanium(known very good antioxidant), polysaccharides(gives long lasting energy), triterpene, Linghzi L Z 8, nucleosides and enzymes(aids digestion). Many atheletes have used reishi, chaga and cordyceps. The beneficial effect of Reishi is probably multifactoral. I suggest that if you want to use reishi, start slowly (400mg) and increase the dosage as your body adopts to the ingredients. Just don't go over board (I would stay under 1500mg per day) although I haven't heard problems of Reishi overdose except due to allergy or starting with too high dosage. If you use medication of some sort beware that reishi might have some sort complications with immunosuppressive drugs or lowering blood pressure drugs although some studies have shown that reishi has improved in some cases therapetically the condition of chronic kidney disease patients taking immunosuppressive and lovering blood pressure (Check Pubmed). Most importantly listen to your body! Regularly it takes a week or ten days for body to adobt for Reishi so be patient the stuff usually delivers :)
Best products of Reishi are probably made by japanese goverment approved Reishi farmers. But I wouldn't buy right know from Japan because its difficult due to language problems and they just had a powerplant accident which may have impacted their reishi harvest. I have used Duanwood, Mushroom Science
Reishi and Chaga and also New Chapters Reishi that I have bought from Iherb.com. If you haven't ordered from Iherb before you can use my coupon code EXI747 and you get 5 dollars of from the deal :)
You can find more information of these products from the companies websites: Dragonherbs.com, NewChapter.com and MushroomScience.com
Beware that some companies make Reishi that is bad quality. Always buy good quality Reishi or don't buy at all.
I have found that Reishi keeps me sleeping better and I can recover faster. It keeps me more calm and it is a wonderful aid in my life busy life. Highly recommend. You can boost Reishi by taking it with good quality vitamin C (250-500mg).
Chaga gives nice stable energy due the day. Don't take Chaga in evening because it can keep you wake.
You can also go and collect your own Reishi (from forest that is not near of pollution) if can recognise the mushroom or can get help from someone who knows the mushroom for sure. Then you can make your own Reishi extracts. I would boil Reishi for couple of hours in water.
Hope this information was useful.