It is a problem, all the adapters I have focus slightly past infinity, which is better than not reaching infinity but still not right..
Having said that, I have had lenses that go past infinity on the cameras they were designed for, but not all lenses do that.

It's cheap production standards in most cases, but many of the expensive ones also have the same reputation. Are we getting ripped off by the manufacturers / retailers of supposedly 'quality products'?

With live view on the NEX and surprisingly accurate focus confirmation on the K10 I have got used to trusting that technology and not hitting the stop for infinity.
It doesn't make it right, but I think it's all we'll get.

I've done a few successful long time night shots with the Pentax K10 and the NEX5 with good results, probably better than my film attempts as there is the luxury of chimping as you take the the shots, and processing afterwards. I suppose its a different approach to getting the final image because I view the shot, then adjust and reshoot until I get what I want. But that's the luxury of digital.
The high ISO's that modern digital cameras attain are astonishing, and that again opens up whole new aspects to the shot we desire, we can shoot from ISO 100 to ISOwhatever without changing film.
Long exposure night photography becomes a pleasure.

Out of 85 lenses I have 20 zooms, most are average but some are very good. There's 3 Vivitar Series 1, 2 70-210's and a 24-48, they are very good, especially the wide zoom, all of them are Kiron, as is a really nice Soligor 70-210. I rarely use any of the 70-210's though as they are just too big and heavy.
The Tamron BBar zooms are good as well, but again I rarely use them.
The only zoom that gets regular use is the superb Pentax DA 50-135 f2, a current AF lens. It's also the only AF lens that gets regular use. It's actually the only AF lens that I have ever bought new because I wanted one, the few other AF's I've got are the kit lens for the Pentax and the NEX and a couple that came with the AF Minoltas and EOS cameras. The kit EOS zooms are junk, I struggled to give one away.

I have a bunch of primes from 24 to 400 in M42, PK, FD, MD, M39, OM, EOS, Sony and RolleI QBM, and with adapters I can use many of them on cameras as diverse as the Sony NEX, Pentax K10, and a whole bunch of old film SLR's.

I find that most old film zooms flare badly, and give a lot of CA on a digital camera, more than film. It's generally easy to disguise or remove in processing, but it's not such a problem with the primes, so they get picked first most times.

I guess I'm a prime man.

The Takumar should be excellent, I find them sharp but lowish contrast as a rule. Easily fixed in PP or the darkroom.
The Mamiya's have a good reputation as well, so that is worth keeping and trying. The Cosinon ? they have a mixed reputation, I had a 2.8 28 for a while and found it didn't perform as well as my other 28's and 30's so off it went. But since then I've read that some people love it. Perhaps there was more variation in the quality of the cheaper Cosinon ?

Soligor never made any lenses, they were an import and marketing company, so much of their product line came from various manufacturers. The 'CD' range was their top quality, competition to Vivitars 'Series 1', and generally very good.. I have the longer zoom 70-210 in that series and it's a very decent performer, the other CD's I have are primes and also up to the same standard. I like Soligors and have 11 of them at the moment.
According to the best knowledge on the serial numbers the maker of that zoom lens is almost certainly Kino / Kiron who were a good third manufacturer and sold a few good lenses under their own names.
For a few bucks, I'd buy it and try it, as long as it's working and not decentered or anything drastic it should be good.

I help a few local charity shops by sorting out donated cameras and equipment, then either I sell the good stuff for them at decent prices or they put it in the shop. And two types of camera we just can't sell? old 35mm compacts and old digitals.
I've shifted a few 35mm compacts to the Lomo lovers, and some of the better compacts - mainly the AF ones - I've still got here. The rest will be binned shortly. But the few digitals are still working and I'm loathed to throw them in the bin. I think there's a Canon Sureshot S40 4mp, Fuji Finepix 2300 2.1mp and a Vivitar Vivicam 3750 3mp left.