I've made the switch, and I'd like to briefly discuss these two lenses.
The main reason for me is having the longer end on the Nikkor. 16mm is still way too wide, and I often had to crop pretty severely. I hate switching lenses, and I don't even have a 18-whatever kit type lens. The better (heavier) ones I wouldn't want to lug around anyway.
A big advantage of the Tokina is the f/2.8 - that's not something I need at all. I shoot f/8 or f/11 >99% of the time. The Nikkor is lighter, which is nice for hiking.
My Tokina had broken AF (an unfortunate run in with the girlfriend) which wasn't that much of a big deal. Easiest lens ever to manual focus. I got a good deal on the Nikkor, so the upgrade only cost me $200.
The lens I would love to use is the Sigma 8-16mm, but sadly it doesn't accept filters. I love using a polarizer for mountain lakes, so it's a big deal for me.
The Tokina is a better lens for the money if you pay retail. It's just not built as tough as it may seem: a 30 cm drop to the carpet killed the AF. If you could ever imagine a survivable drop, that was it.
Playing: tired from volleyball tournament
Eating: tasty salad