I'm starting to use it.
The major side effects or concerns about Niacin are two
1) Insulin resistancce
2) Reduced levels of methyl donors and an increased levels of histamine and serotonin
for 1) you can take niacin bound chromium, or change your diet and eat less carbs and doing more exercise. for 2) you can take TMG/Betaine.
What I'm doing for now is to take Niacin before bed. It makes me sleep a lot better, don't know why but sometimes I wake up refreshed after just 3 hours of sleep. Niacin before bed has other benefits, it is known that increased insulin levels reduces HGH pulsatile effects, basically low insulin enhances HGH, such thing happens with fasting, If you don't know try to search about it, muscle is preserved while fasting due to HGH increase.
Niacin produces glucose store release the first hours and stops FFA release, but then it produces a rebound effect in FFA, apparently (I have not tested yet) Niacin increase ketone bodies production, so if you take Niacin before bed and if you sleep 8 hours you will have increased ketone bodies because you are not eating food in that sleeping window.
So Niacin before bed enhances HGH, increased ketone bodies production and reduces the potential develop in insulin resistance because you are in an empty stomach for 8 hours or more.