I've had one since 2004. It's ok, just ok. Heavy, a bit soft for a prime lens, lots of ca in the corners. 17mm is the perfect focal range for me, but if I had it to do over again I'd pass. A 10-22 may be a bit more expensive, but it kicks the dogshit out of the tokina any day.
save your pennies for a canon 15mm (or the 10-22mm). if you cant afford it then get a sigma 15mm. if either of those are not worth the amount you make in tandems, then just keep the kit lens. i have had at least 7 magazine photos with the kit lens. it is just fine.
I found also reasonably cheap used Canon 10-22, just doing the $$$ math
Do the weight math while you're at it. The 15mm weighs about twice as much as the kit lans, and the 10-22 is another ounce on top of that. Also consider the size, the 15 is smaller than the kit lens, but the 10-22 is bigger than both of them, so the extra weight is hanging further out from your head.
The body is as important to a photographer as a canvas is to a painter. The lens is as important as a brush is.
You already have a good canvas, changing to a sligthly better one wont make your paintings much better. However the brush you are using is too thick for tandems IMO. Its difficult to paint well with a brush that is too thick. You need the right tools for the job.
Changing from 18mm to 17mm wont make a difference, but the 10-22 will give you a variety of brush tools to choose from. Definetly go for the 10-22.
better glass is more critical then updating the body (in this case)
if you were considering a 5D MkIII then it would be a different story... but in this case with the smaller frame (APS Sensor) you probably won't see a substiantial difference between the 400d and the 600d.