This is great for crop shooters. However, before everyone gets excited over the f/1.8 bit, you have to remember that f/1.8 on a crop sensor is nothing like f/1.8 on a FF sensor. This lens will give the same angle of view, image noise for given exposure parameters (*1), depth of field at a given AOV and subject distance (*2), etc. etc. etc. as a 28-50mm f/2.8 full frame lens.
In other words, if the lenses and sensors are perfect, this lens on a crop sensor would give identical results to a 28-50mm f/2.8 on a FF sensor. However, lens and sensor imperfections actually favor the larger format sensor, so don't expect this lens to give anything as good as the 24-70 f/2.8 original or the tamron.
Still, its great improvement for crop shooters (if it keeps up with recent Sigma trends), and should be relatively compact.
(*1) This considers photon shot noise only and assumes photos are rescaled to same resolution when printed.
(*2) At non-macro distances