Originally posted by paperbag846 A SMC Takumar (or SMC - K mount) 50mm f1.4 is an absolute steal at the price, and being able to mount them on a modern DSLR (with such ease) is *not* possible with Canon or Nikon.
This is not a good example, because the 50/1.4 can be mounted without any issues on any Canon camera. Canon has good compatibility as long as you get an appropriate adapter, because their EF mount is wide and has a short registration distance.
Nikon fares worse because of their longer registration distance which requires an optical element in the adapter to maintain infinity focus. It's really the worst mount as far as adapting lenses from other systems goes.
The ability to use any old K-mount lenses without any adapters is really what the backward compatibility is about. It would be great if K and M lenses would work perfectly well, but like Anthony pointed out, the compatibility is limited in their case.
To review, Pentax is better than Nikon - Nikon has the most limiting mount in terms of adapting other lenses. Canon fares very well - as long as you get chipped adapters, the cameras will handle the lenses well - but you can't easily reuse adapters with multiple lenses, because they need to encode the focal length in them. And Pentax provides a simpler adapter story that has limitations in terms of metering, AF confirmation, and ability to switch lenses, but it compensates by allowing a bunch of lenses to work with a single adapter and make use of SR.
No system is really perfect, but Canon and Pentax do pretty well. As for adapting Pentax lenses on Canon, it's not that easy, because the aperture flange of Pentax lenses can interfere with the mirror movement on some Canon cameras. Even some M42 lenses have issues when used on Canon cameras (Mir 1V, for example). BTW, Leica R lenses can be adapted on Pentax - you just need to get a special K mount piece, unscrew the Leica R mount, and screw on the K mount - the process is not destructive and can be reverted if needed.
In the end, if one is an AF user, ability of adapting other lenses becomes irrelevant, because AF is lost in the process.