***I have no experience with any of the mentioned products merely an educated guess based on the personal opinions I have read and my educated guess/opinion based on the looks of the products***
I vote for Hardrace their products look much more solid. A function > form type product.
Here are the front upper control arm style camber adjusters from Hardrace ($175.00):
Pros from what I can tell: Solid (as in looks strong as hell, not solid metal, it may be hollow for all I know) bar construction, the plate holding the ball joint is thick. The welds look alright from what I can tell. Hardened rubber bushings which from what I have read hold up pretty well. Beefy hex bolt hardware. Overall looks extremely robust.
Here are the front upper control arm style camber adjusters from Blox ($140.00) and Skunk2 ($206.99) respectively:
Stamped metal construction, metal overall seems thinner than the Hardrace units. Does not come with everything you need to install (you have to reuse old parts). Use wussy allen head bolts which I personally have a habit of stripping out, especially after they have seized up some. I have read that the S2 ball joints do not last very long.
Just the visual comparison I would put my money on Hardrace to outlast either Skunk2 or Blox. The Hardrace unit is also competitively priced as well compared to the others. I got all my pricing and photos from JHPUSA.com but do not have a personal bias for or against them. They just have really good product photos.