I recently purchased a Promaster EN-EL15 (which I believe are rebadged Delkin's) and so far experience no issues and similar performance to Nikon brand.
Before that I used Promaster battery for my D80 which actually had a higher amp/hr rating then the Nikon with no issues. In fact the step-granddaughter is still using the Promaster battery some 7years later with the D80 I handed down to her.
You'll here a number of people caution against anything but a Nikon battery. Personally I have had no issue with the Promaster's, but I would be hesitant to order any unbranded batteries off of e-bay, for those are the ones you generally hear horror stories about.