Here are the reason why I chose something different: the helicoid with a M42 mount, and 52mm opening: 1. Some of my lenses have very large rear openings and 42mm is not large enough to accept the lens, and a 52mm opening gives me a better chance that the lens would fit. 2. I choose the M42 mount because it's interchangeable. You can buy the helicoid with specific mounts like NEX E-mount, M43 mount, or Canon EF mount, etc. With an M42 mount, I can use this helicoid on more than one camera system (I have NEX, M43 and EOS mount cameras), by simply screwing the helicoid onto an M42 adapter, or a c-mount adapter with M42 inner threads. It has its compromises, of course. For one thing, when used with enlarging lenses, I would need a M42 to 52mm step-down ring. This would increase the thickness of the helicoid by about 3mm.
The picture below shows how I normally connect the enlarging lens to my camera. For this particular lens (Rodenstock 50mm f2.8), it works almost perfectly, infinity focus is just slightly beyond infinity. So, what happens if your enlarging lenses are longer than 50mm? Very simple, just add 52mm filter rings. The last picture shows an 80mm enlarging lens with 3 filter rings added to compensate for the longer focal length. For very long lenses, like 150mm, it's better to buy a longer helicoid, or use extension tubes instead of filter rings.
It maybe worth mentioning that the c-mount to NEX adapter with inner 42mm thread is now hard to find. So, it maybe be better to buy a helicoid with the mount of the camera you are using it on, instead of an M42 mount.
How the pieces are put together. Click for larger.
All connected. In real life, the labels won't line up nicely like the picture. Click for larger.
Mounted to the camera. Click for larger.
With extension rings (filter rings) for longer lenses. Click for larger.