The newly announced Samyang 135 1.8 is getting good reviews, £798 in the UK vs the £1500 for the Sony GM version.
Don't think there has ever been a bad 135mm lens ever made and the 135GM is exceptional.
Samyang lenses are often well reviewed as they will always take the very cheap cost to buy into account. Samyang are also very good in terms of providing free equipment and incentives for reviews, they are the best at this from any of the manufacturers. Even with a small Instagram account like our own (we have 15k followers) they have been in touch several times via their marketing company offering us free equipment in exchange for a review. Free stuff generally = a positive review.
Samyang's quality control though is absolutely shocking and it's a complete crap shoot in terms of getting a good copy of every autofocus lens they make, (Some of their manual focus lenses are excellent.) I guess that is one way they keep their costs down.
Samyang lenses are a good choice for the amateur photographer that doesn't need to rely on their equipment. My eldest has a few Samyang lenses which she is happy with, but to be fair she doesn't really know any better as we don't allow her to use the gear we use for work.
We have owned quite a few Samyang lenses, who doesn't like a bargain, the only one I was ever mostly happy with was the 45mm, although even with that one, the first one that arrived with us had several large clumps of dust inside it and had to be returned for replacement. All of the others had either permanent or intermittent a.f issues or were decentered. Add the orange colour cast that effects all of their lenses and even though they are cheap they aren't for us.
I can see why Samyang lenses will appeal to some. If I wasn't reliant on my equipment for work I could probably live with some of the common Samyang issues to save some cash. When buying equipment for work the price doesn't matter just as much, it's more important to get the right tool for the job. Although that's not to say that the price doesn't matter at all of course.
The other simple truth is that some people just can't see the issues that would stand out like a sore thumb for others. Just as an example, my wife is very friendly with another wedding photographer, she had a Zeiss Distagon that she has been using for donkeys years she pretty much shoots everything with it. She is an ultra trendy hipster wedding photographer and they all pretty much only use a 35mm lens. She has shot probably about 500 weddings using the distagon and not being the most technical person when it comes to equipment never realised at any point at all that the lens was very badly decentred and has been from day one. My wife wouldn't be the most technical person either but even she realised straight away that her friends lens was decentered. You wouldn't believe how happy the other photographer was after my wife lent her our 35GM for a couple of weddings. She had been thinking for years that the issues she was having were down to user error, she had never even heard of decentering.