The thing at the end is the Ubuntu "distribution" aka "release" (for that v4.1-rc2, "vivid vervet" aka 15.04). So in this case "unstable" really refers to the ubuntu "distribution channel", not the kernel itself.
But really, you should expect it to take at least a few days to migrate to a more "stable" "distribution channel". On Arch Linux, they often don't migrate it before the "x.x.1" release. That's because they make sure various out-of-tree kernel modules are still working, and they sometimes do need to apply a patch to fix a strange issue related to some hardware that's being used slightly differently, and isn't caught until after the "x.x.0" release which gains much wider usage than the release-candidates.
Ubuntu isn't even a rolling-release distro like Arch is, so what you linked to is a special repo you can add for super up-to-date kernels if you really want them for some reason. But if you have no particular reason, you shouldn't bother. There can always be unexpected complications, usually involving closed-source kernel modules (nvidia, amd catalyst, vmware) or buggy hardware (but of course a linux enthusiast would say that ;)
http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.1-rc2-vivid...
The thing at the end is the Ubuntu "distribution" aka "release" (for that v4.1-rc2, "vivid vervet" aka 15.04). So in this case "unstable" really refers to the ubuntu "distribution channel", not the kernel itself.
But really, you should expect it to take at least a few days to migrate to a more "stable" "distribution channel". On Arch Linux, they often don't migrate it before the "x.x.1" release. That's because they make sure various out-of-tree kernel modules are still working, and they sometimes do need to apply a patch to fix a strange issue related to some hardware that's being used slightly differently, and isn't caught until after the "x.x.0" release which gains much wider usage than the release-candidates.
Ubuntu isn't even a rolling-release distro like Arch is, so what you linked to is a special repo you can add for super up-to-date kernels if you really want them for some reason. But if you have no particular reason, you shouldn't bother. There can always be unexpected complications, usually involving closed-source kernel modules (nvidia, amd catalyst, vmware) or buggy hardware (but of course a linux enthusiast would say that ;)