One of the main reasons I got on the mirrorless bandwagon with the Sony a7ii is so I could use legacy lenses with the M39 and M42 screw mounts. After finding some candidates, I searched Flickr for groups dedicated to them, choosing the lenses based on the results others were getting with them.
These included 3 Soviet lenses, seen below in the order: Industar-69 28mm f/2.8, Industar-22 50mm f/3.5, and the Helios 44-2 58mm f/2. All three were picked up on eBay from Ukrainian sellers. All three cost less than $50 each (including shipping) and all arrived in decent condition. The only catch was having to wait 2-3 weeks for them to reach me in Washington, DC. Some first impressions:
Industar-69 28mm f/2.8: Pancake wide angle lens on an M39 mount. What's great is that the profile is so low that I can keep the a7ii in the pockets of my larger coats while using it. I find it to be way too soft at f/2.8 (though that could be just the one I got -- you never know with inexpensive vintage lenses what is or isn't due to an issue with your individual copy), but it gets nice enough at around f/4 up. I get excessive vignetting with my adapter -- unlike my Fotasy M42 adapter, the length of my Fotodiox M39 adapter is not adjustable, otherwise I'd lengthen the adapter when using it with this lens. So uncropped daytime shots can look a bit silly...
But for lowlight and night shots, the vignetting can fit in very nicely...
Industar-22 50mm f/3.5: M39 mount. Aesthetically this is the coolest lens I've ever owned. Yes, having a steampunk-y chrome vintage Soviet rangefinder lens adapted onto your digital camera is exactly the sort of thing hipsters get mocked for, but you can't look at it and not love it. And I really like the way this lens handles. I like focus levers -- the one on this one will accurately lock at infinity and has a push button release that works well. The lens barrel itself locks in place at full extension when in use, but is collapsable for storage....
I try to keep it always locked forward when it's on my camera -- it's rather scary knowing a jagged ring of steel is shooting back within your camera right in front of the sensor (or in the Sony a7ii's case, the anti-aliasing filter over the sensor). To make sure this $25 lens wouldn't ruin my $1700 camera, I measured using the a7ii's sensor plane mark, and found there's a small window of safety.
The performance has been a nice surprise. It's fairly sharp across the board, the vignetting isn't bad, and some of the lens flaring can be quite pleasing...
I'm also pleasantly surprised by the sharpness. A long exposure night shot, taken somewhere around f/6...
Not totally up to par with my brand-new Zeiss, but again, keep in mind that this lens was had for less than $50!
Helios 44-2 58mm f/2: This one goes on the longer M42 mount. Optically, it's also a terrific bargain...
It's also taught me how to use preset stop-down lenses. These are lenses with 2 aperture rings -- one that clicks and sets your minimum aperture, and another non-clicking ring that actually controls the aperture. The point used to be that if you wanted to shoot at, say, f/8, then you could set the clicking ring to f/8, and then set the controlling ring to wide open (in this case f/2) so you'd have the most light for focusing and composition. Then when you're ready to shoot, you could quickly turn the aperture control until it stopped at you desired f/8.
That aspect is anachronistic for most of us, but I've found I really like it otherwise. Most modern manual lenses have clicking aperture control rings -- the advantage to clicking in place is that you can be sure of the exact aperture and you're less likely to accidentally change it. But one disadvantage is that you usually can't set it between stops (say at f/3 or 3.1 ish). And if you're shooting video it makes it tougher to change the aperture during the shot because of the lens vibration of the clicking, as well as the noise if you're also recording audio. This is why you'll see some hacked "de-clicked" lenses out there. This double-ring setup (which still exists on a few still-in-production lenses such as the Helios 40-2 85mm) gives you the best of both worlds -- smooth aperture control with one ring and the exactness of a click-stop with the other.