My Rokinon 14mm f/2.8 is pretty good, but I think I've only ever shot it wide open once or twice. I usually set it to f/8 (with a tripod) and fire away. It's a very fun lens.

I am definitely interested in the Rokinon 12mm fisheye (I also have the 8mm fisheye for APS-C and it's a blast). But I'm even more intrigued by Tamron's latest announcement - the full-frame compatible SP 15-30mm F/2.8 DI VC USD Lens.