NIAGEN, also known as nicotinamide riboside (NR), is just a member of the vitamin B3 family. It's just a B-vitamin. Yes B-vitamins are important for energy metabolism but they are very wide spread in foods and I highly doubt you will notice anything at all from supplementing with niagen.
I agree with @stevesmi
I would just get a good quality multivitamin like ruthless 3d from n2bm.com. It'll be only slightly more expensive then a niagen supplement and that way - you know your entire vitamin and mineral needs are taken care of. Not just vitamin B3