What's the story?
While Leica was busy moving on to its M-series cameras with the new bayonet lens mount, Canon was still focused on Leica's older tech. The 7 and 7s are perhaps the most advanced cameras ever to use the classic 39mm Leica threadmount.
Why should I want one?
Actually, the biggest reason to want a Canon 7 is a lens. The 50mm f0.95 "Dream" lens is legendary for that wide maximum aperture – and due to a proprietary outer fixing, it'll only fit the 7/7s. That's not to say the 7 isn't worth seeking out in its own right, and its ability to take the older Leica lenses is a bonus. It's solidly built and typifies '60s chic.
What to look out for
As with any classic camera, check the shutter's firing at all speeds. The slow speeds often get sticky from disuse and need an expensive service. The meter in the 7 is also likely to be dead or inaccurate, and might need the selenium cells replacing if you want to use it. The 7s used a CdS that may need recalibrating for use with modern batteries.