>>Be careful with lower priced SZ series, they are not very well mechanically.<<
I have a SZ-X 28-70 2.8-4.3 and it's the lens I use most often when traveling. This lens has been praised in other forums as a VERY capable optic. Mechanically, the only concern is a polycarbonate front focusing ring. The remainder of the structural components are metal. Mine has performed without issue for 10+ years.
Like all lens brands, there are some quality variations, not only from model to model, but from lens to lens. I think it fair to say that all things considered, Tokina is one of the better third party brands.
Dave
(I also have the RMC 17mm 3.5 and it's a good lens for the price. Above average but no better really.)