I have a couple of Ex-Pro batteries for my Fuji X100T and they are fine, or at least no worse than the underpowered Fuji original. I've never managed to exhaust the battery in a Nikon dSLR in a single day, so I generally have only a single (genuine Nikon) spare, only buying a third party battery for my D70 when Nikon recalled the original and replacements were in short supply. Check reviews for the specific battery you are interested in, and avoid suspiciously cheap camera-branded batteries sold by ebay or Amazon traders. As Jon suggests, these will just be generics of unknown quality 'masquerading as genuine' - much better to get a generic from a trusted third party brand.