In render settings be sure and check "Panoramic View" (360) AND "Spherical Render" (180).
Choose a render size that's big like 2048 x 1024.

As Jim says, don't use Natural film response. Be sure and save as .hdr.
The goal is to have as large a dynamic range as possible, so you want bright whites and black blacks-- if possible. Radiosity, GI, or AI is typically not necessary-- neither is good anti-aliasing as these will all take much longer to render.

Those with multi-core processors will only use 1 core when rendering panoramas. HTH.