>I would like to know whether anybody else experienced the same issue?  
  
To answer this question see the [customer reviews on B&H Photo Video][1] for this lens with a Nikon mount.  
  
There are 69 reviews and the average customer rating is 4.5 stars out of 5 stars.  
  
Of the 69 reviews 3/69 noted that the lens was soft at maximum aperture at the edge. 1/69  complained that the lens was very soft and returned it.  
  
The comments make it clear that this is a highly regarded lens. The main issue reported is focus problems - 8/69  
  
>I would also like to know whether Tamron's equivalent 17-50mm f/2.8 lens is any better?
  
To answer this question see the photozone tests conducted on these lenses, **both on the same camera**, a Nikon D200 (so the results are comparable).
  
+ [Sigma 18-50][2]  
+ [Tamrom 17-50][3]  
  
The results are reproduced below (all measurements in LW/PH ) 
![enter image description here][4]  
From this it can be seen that at 17(18)mm and F2.8 the Sigma lens is much softer in the corners, but comparable elsewhere. At 17mm(18) and F4.0 the Sigma lens is somewhat softer but is comparable elsewhere. In all other respects the lenses are comparable.  
  
In summary then, customer comments indicate the Sigma lens is a highly regarded lens that has some focus problems. Measurements show that the lens is rather soft in the corners of the image at maximum aperture and short focal length. At other apertures and focal lengths it performs well and is very similar to the Tamron lens.
  
Just to clarify things, the image below shows what photozone mean by their terms centre, border and extreme: (blue=extreme, green=border, pink=centre)  
![enter image description here][5]  
[Image on photozone.com web site][6].


  [1]: http://www.bhphotovideo.com/c/product/518483-REG/Sigma_582_306_18_50mm_f_2_8_EX_DC.html
  [2]: http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/315-sigma-af-18-50mm-f28-dc-ex-macro-review--lab-test-report?start=1
  [3]: http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/290-tamron-af-17-50mm-f28-sp-xr-di-ii-ld-aspherical-if-nikon-test-report--review?start=1
  [4]: https://i.sstatic.net/xdWaV.png
  [5]: https://i.sstatic.net/SsHoP.jpg
  [6]: http://www.photozone.de/lens-test-faq