sgallagh, need more data please. Find the first oops in your logs that isn't tainted. You're the only person I've heard of that has troubles with this kernel so far.

This update contains the TSX disable change that Intel released for Haswell CPUs because TSX is broken. glibc in F21 defaults to using TSX for lock elision and it will explode if the microcode update is applied after it's running. The main solution is early microcode loading. Fedora isn't setup yet to do early microcode updates for F21 because we need to change things in the kernel and in the creation of the initramfs by default. See bug 1083716