I use a Pentax 10-17mm lens, which from what I understand is basically the same optical formula as the Tokina 10-17mm (Pentax & Tokina share some lenses like the 10-17mm fisheye, 16-50mm F2.8, and 50-135mm F2.8, all of which are Pentax optical designs available either as Tokinas with their own lens coatings and barrel design, or as Pentaxes with Pentax's proprietary SMC lens coating and their own barrel design (weather-sealed in the case of the 16-50 and 50-135.)
That's a long way of saying that my fisheye shots are virtually the same as you'd get with the Tokina lens on a non-Pentax DSLR. (Potentially a little more lens flare on the Tokina due to the different coating.)
As is probably obvious, I'm a big fisheye fan.

The UWA rectilinear lenses don't really do it for me. 16mm on my APS-sensor DSLR is plenty wide enough for me for non-fisheye shots; after that, I prefer not only the look but the flexibility of the fisheye. You can get quite a variety of "looks" depending on where you point it, and the zoom lets you get photos that aren't all that removed from an UWA shot in terms of distortion - plus you can mostly remove that in PP, but you can't take a non-fisheye and make it fishy. My only complain is that it's not F2.8, but that's the price you pay for it being a zoom.
I'd actually really like to try the new Samyang 8mm fisheye but would have a hard time justifying the cost... but it is a good bit cheaper than a new OEM fisheye.