I bought two of these batteries to supplement the one I already had. I specifically bought this product because in the advert it was billed as a Nikon original EN - EL5 battery.
The batteries arrived in Nikon Packaging and look exactly the same as the original supplied with the camera including the hologram sticker.......except they are a slightly different colour being a little darker. This aroused my suspicions enough to weigh them. The original supplied with my camera weighs in at a whopping 26.81g and of the ones I bought from here, one weighed in at a substandard 23.16g and the other is 22.76g.
I haven't needed to use them yet and so cannot comment on their performance, however I suspect they will prove to have a shorter life span than the original, as either they are very good but expensive copies, OR they are indeed originals which have somehow had a cell or two removed.
I would imagine that they will work fine but need charging sooner than the original, and I have to admit that I wouldn't have bought them if I'd known they weren't exactly the same as the one I already had!
Edited 1 Nov 2010: I feel I must edit this post as I would like to urge Amazon to stop selling this specific item. I have been using the two batteries that I bought, and for about six months have had no problem, however the other day, as my camera showed low power I attempted to remove this battery in order to recharge it. But I couldn't get it out of the camera as it had swollen inside the camera. I eventually managed to remove it but it was so bulging that I couldn't bring myself to charge it. I decided to throw it away and charge the other one that I'd bought at the same time, but found that that too was getting bigger. They both went in the bin and now I'm just down to the original battery - which is performing fine. I would advise everyone to steer well clear of this item and NOT take the chance. DO NOT BUY. :-(