I own the 12-24mm/4. I have only had it for a few days, and can't say much yet.
What I can say, however, is that it's well built, all metal it seems (except the rubbery grippy thingy on the rings). The sun cap sits nicely on and is easy to put on. Thus far, the weird tokina style manual to auto focus thing has worked nicely. The rings are smooth, and the focus ring is big, which is good. Auto focus is decent, but quiet.
I'm assuming the 11-16mm/2.8 is about the same when it comes to construction and auto focus speed and such. Some say it's a tad sharper than the 12-24mm I realise these aren't primes, but worth looking into... unless weren't considering something this wide.