To make it short, I settled on Tamron 28-75mm f2.8. It has a constant f2.8, which is fantastic and one of the reason I didn't choose Sigma's 17-70mm. Plus, I like the extra 5mm and don't really need the wider range. The problem is that it seems that it's quite common to get a "bad" copy of the lens. I really dreaded that because Indonesia's return policy is non-existent. But the photos I saw in the Tamron 28-75mm's flickr group is gorgeous and the review of the lens is generally good. So I decided to give it a try.
The main complaint about the lens from what I've read is that front and back focus problem so I went to the camera store prepared with a focus test chart I downloaded from here: http://www.focustestchart.com/chart.html
I conducted the test without a tripod and without the perfect lighting, but I wasn't too concern about it. I'm not looking for a tack sharp enough but just tested to see that it can focus where it's supposed to focus.
The results seemed to be quite good:
I also took a couple of random test at different zoom range:
So all seemed satisfactory and I bought the lens and brought it home. I'm really loving it. I think I've found my near perfect lens.