The 8MP is no gimmick. It provides 60% greater image density and that is substantial. It is the equivalent of upgrading from a CP990 to a CP5700, percentage wise. If you only shoot for the computer monitor - web work and slide-shows - it is of no consequence. However, if you shoot for prints it is QUITE significant. Even for web-work, it allows a much greater degree of cropping with no loss of quality. All else being equal, more pixels is better.
It is a gimmick if the pixels are so small as to provide worse quality than less pixels. This is the trouble that the Sony 828 has: you can see better resolution but you see it through lots and lots of noise as well as record-breaking chromatic aberation. IMHO, If Nikon uses the same Sony sensor, it will not be a very good camera.