I know for me, when I bought mine (and to be honest I mostly still do this), I bought it so I could use it on film and digital, and there were no other choices "out of the box" that could be used across platforms. Honestly 12mm on FF is even more wild than any fisheye and I'm a big fan of fisheyes. Plus the Toki 11-16 hadn't come out yet I don't think.
To say that there are 10 better options than the 12-24 would be a stretch. Lets not forget there are few lenses as well corrected, well built, and include HSM/USM + FTM in an UWA... in terms of those features, I can only think of the Canon 10-22, if you take away the USM/HSM w/ FTM feature, you're down to the build of the Tokinas and nothing else to compare it to - and the Toki's are built more solid than the Sigma, the only lenses to be able to say that. But as many will chime in, its hard to pick a "bad" UWA for a crop these days - its arguably the hottest lens segment of the last couple years. No other focal ranges have so many lenses to chose from specifically for crops as the UWA segment. So, with all due respect I say theres 3 better options, and those would be Tokina 11-16 f/2.8, Tokina 12-24 f/4, and Tokina 12-24 f/4 II. I don't consider the Canon a better option - I'm just not a fan of that lens.
when it comes to FF, where the Sigma 12-24 calls home, Its definitely a truly unique lens, a lens with no equal. I know when I'm shooting mine on film I spend most of my time shooting between 12 and 15mm, with MOST of that time @ 12mm. Its just a perspective that can't be replicated by any other lens. The closest one can get to that shot is with a TS-E 17 f/4L.
I'm surprised Sigma ran the price up of the lens tho, seeing as its never been a super big seller. In a hotly contested market segment such as it is in, such sticker shock is going to hurt sales further... I don't understand why they'd do that.