Lowest price ever according to the camels - RRP is $539 (although Amazon says it's $549)
A decent ultra-wide (in full frame/35mm terms, anything wider than 24mm is 'ultra wide') lens - although personally I'd call it something more like a 'super-wide'. Equivalent to a 30mm or so if for some reason you put it on an APS-C camera. Anyhow yes, it would be nice for it to be f1.8 rather than f2.8 if you want to use it for astro photography, yes the barrel distortion from it is awful so if you plan to use it to take architectural shots you will need to do post processing which in turn means losing some resolution, and yes the focus motor is apparently noisy so not great for video plus there is no image stabilisation. Did I mention vignetting? If you want a better lens, check out the sony sel20f18g, but expect to pay about $1,000 more for it.
On the plus side the Tamron is well made, has weather proofing, is small and lightweight, can take sharp photos with decent contrast, and is cheap. If you understand what the limitations are with the lens, you can work around them for the most part. I was thinking about getting a Rokinon/Samyang 18mm (for landscape and some astro shots), but they cost more and apparently the QA on them is marginal with many reports of bad units. I've had good experience with Tamrons in the past (their 90mm was/is a gem), so I ordered one a few days ago when it hit the lowest price ever, but was waiting for it to arrive so I could check it out myself before listing it as a bargain. It dropped in price again today so thought what the heck - someone else might be interested!
Again, this lens is far from perfect but for the price I think it is worth considering for some.
Lots of reviews out there, eg:
https://www.dpreview.com/products/tamron/lenses/tamron_20_2p…
https://www.the-digital-picture.com/Reviews/Tamron-20mm-f-2.…
https://www.digitalcameraworld.com/au/reviews/tamron-20mm-f2…
https://www.youtube.com/watch?v=Vw-zz9eLxnE
What's the focus speed like? I have an A7 mk1 and the Sony nifty 50mm but the focusing sucks big time.