Such lenses are easily available for a few dollars (just search for 'magnetic lens' in popular e-tailers) and can make shooting with the Nexus 5 much more fun.
Also, as big as the camera ring is, the magnet is still not all that strong, so while most lenses seem to fit fairly well, bigger ones could easily slip off or not fit at all. This will thus find best use with smaller lenses.
The last disclaimer to all this is that the brilliant idea is somewhat flawed by the lazy auto-focus of the Nexus 5 that becomes even slower to lock focus when used with a third-party lens. Ah, Google has told some publications that a software update aimed at improving the camera is coming, but it's not known whether it'd fix that.
Still, the magnetic ring is a nice touch that went under our radar initially. It's this kind of attention to detail and small touches that we find particularly exciting, even if discovered a bit late. Right below are a few images of the Nexus 5 with a snap-on lens and the samples shot with it.
Nexus 5 camera ring magnet /all images courtesy of Patrick Schroedl
2. Nexus 5 camera sample without a lens
3. Nexus 5 camera sample with a wide-angle snap-on lens
4. Nexus 5 camera sample without a lens
5. Nexus 5 camera sample with a wide-angle snap-on lens
6. Nexus 5 camera sample without a lens
7. Nexus 5 camera sample with a wide-angle snap-on lens
source: Android Central, XDA Developers