I don’t have much experience with fisheye glass, the only few I ever got were with Canon EF 15mm F2.8 fisheye down at the Dallas city rookery. Granted, a 15mm is too wide for birdies but definitely not enough for panorama freaks with crop-factor DSLR. If 5.6mm is wide enough for you, Sunex just launched a SuperFisheye at PMA 2008.
This Sunex SuperFisheye 5.6mm F5.6 circular fisheye lens has an angle of view of 185° in all directions, provides enough overlap and allows stitching software to realize a 360° immersive image with only two shots. It is exclusively designed for digital SLR for Canon APS-C and Nikon DX format DSLR Cameras to produce a full circular image within the frame.
The lens kits costs about $799 including the Dewarper DSLR Photoshop compatible plug-in filter for correcting or optimizing distortions that also works with other fisheye glasses like Peleng 8mm, Canon 15mm or Sigma 8mm. As for me, I got my eyes on the newly announced Sigma 4.5mm F2.8 and you'll be the first to read about it when the day arrives.
Rookery shot with Canon 15mm F2.8 Fisheye