If you have a 32 bit device, and are jailbroken on any iOS including the new 9.3.5 device. You can install any iOS from 6 to 9 if your device is compatible.
If you have a 64 bit device running iOS 7 to 8.4 you can also dual boot iOS.
I tried it and it runs very smoothly. I think this is very kool. You can easily switch between the current os and the other one you have installed. You can even uninstall the os if you just want to stick with just one.
Some people say that jailbreaking is useless because apple has implemented everything in the current iOS. I say no.