Tamron have released two new primes:

35mm f/1.8 VC

and

45mm f/1.8 VC

Lenses look (theoretically) good .. as in the specs look good, not referring to a visually pleasing design!
MTF graph looks nice for astro types, with minimal aberrations.

Bokeh looks good from the (Tamron) sample images posted on the 45mm
I'm not so sure on the 35mm tho. The sample image of the woman on the pier looks OK except that the highlights in the far distance seem to have a not very well rounded look to them.

The image stabilisation is a handy feature to have, and the lenses are smaller and lighter than the Sigma Art equivalents. (Sigma's are obviously faster at f/1.4 vs f/1.8 on these new Tamrons).
Tamron reckons about US$599 for each, which is a little too high if compared to Sigma's very well regarded Art lenses .. but as we all know those prices will fall a little in no time at all.

So the question is cheaper smaller lighter and with VC, or Sigma's faster Art lenses as alternate options.

Had the prices been centered more around the US$300 region .. for me it's a no brainer. For not much more than the manufacturer equivalents PLUS image stabilisation .. I'd have them both in an instant.
At the quoted US$599 price range, they are in the F/1.4 lens region, and even with VC I reckon it makes them something to contemplate as opposed to an instant decision.
(I reckon) prices more like US$500 for the 35mm and $US300 for the 45mm would make more sense.

Anyhow, looking forward to see how they fare in the real world tho.