I've being using a Canon EF 15mm f/2.8 & a Samyang 8mm f/3.5 (diagonal fisheye made for APS-C bodies) for a while.
Photos of bands, both on stage and in the street, and using it as an ultrawide lens got me great results and positive feedback. I can get a whole scene, say the band playing + people dancing + crowd watching, and people like it. I didn't have to correct it, and nobody commented it's distorted or gimmicky.
I've used the EF 15mm f/2.8 to take photos of nature lanscape. Here I have to very carefully compose and correct the distortion, but the results are panoramic and impressive.
I've recently bought a Sigma 8mm f/3.5 circular fisheye for use on my 5Dmk2, and would appreciate advice as well.