Yes - I have this lens. It's certainly a keeper. Although... I must admit, I haven't used it in a long time. It got replaced by the 31mm. And since then, I now do the zooming with my feet!
The wider end got replaced with a FA 20mm f2.8 - which I absolutely love.
There are a couple of different versions. I have the 77mm filter version, fluro orange band at front, focus distance markings on zoom barrel (so no window). Metal construction - very solid build - built like a tank. (Heavy.) Mine also came with the Tokina BH-773 petal hood. About 7-8 years ago, everyone was going gangbusters over this lens. The rumour was that it was sharper than the Canon 20-35mm lens - at half the price. In fact, there was talk of little IQ difference to much more expensive Tokina AT-X version that came out later - about the time this lens was discontinued. It got replaced by a lesser (in build and IQ) 19-35mm version. The thing about the Tokina 20-35mm AF was the sharpness. And it is very sharp. (This was surprising for a wide angle zoom lens.) Stopped down to f8 - it is tack sharp in centre as any prime / and provided decent corner to corner sharpness. The sharpness is very acceptable wide open too. There are noticeable distortions at either end - especially at 20mm. I found that if you didn't push it wider than say 23mm - everything was okay. (This helped when taking Architectural shots.)
On FF - this lens was pretty amazing. I spent a couple of years with it being glued to the front of my trusty MX - it was the ultimate landscape lens - shooting Velvia-50. The colour isn't bad - but Pentax shooters are a little spoilt in that department. All non-Pentax SMC lenses can fall a little flat sometimes. Actually, it is a very contrasty lens - but you need to be weary of flare - which will wash out the colour and contrast.
For some reason - my copy has a tendency to over-expose. Although, that is easily compensated for. I remember that it can focus really close too. What I always loved was how bad-ass it looks on the front of any Pentax camera. In fact, I just pulled it out and stuck it on my K20D. Seriously, that's a mean looking piece of kit. I think you've just inspired me to pull it out of retirement and go and shoot with it.