I just purchased the Tamron 28-75 to compliment the FE 70-200 & FE 200-600 mm. Just looking for a wide angle Landscape/astro-photography at the bottom end and maybe the 85mm (and I'll be happy).
The Tamron 17-28 seems decent for landscape. The Sigma 20 1.4 is beautiful, but I haven't yet tried it for astro - just not enough darkness for the next few weeks.