I do not think it is a gimmick because in some games, it does enhance gameplay. For example, in Ocirina of Time, the landscapes look beautiful in 3D, and it brings a great sense of depth. In other games, however, it is a distraction and should be turned off. Also, the many features the system comes with are neat and kept me occupied for longer than I expected. They do great to show off the effect and make you want to use it more.
If developers really take the time to make it look right, than it is a worthy addition to gameplay. Outside of that, I guess you could call it a gimmick. In the end, it is all a matter of opinion. Some like it, others don't.