It is a rare camera and there are two types of it. 
Model Description Edit
Zenit 18 is a follow-on in the Zenit series of the Zenit 19, using mostly the same SLR body, but omitting the DOF preview button, and transferring the activation of the light meter from it to the shutter release. This camera also offers aperture-priority automatic exposure, a significant first for Zenit M42-mount cameras. Even more significantly the camera allows full-aperture metering with special M42 lenses equipped with proper electronics and electric contacts.
Other differences to 19 are faster flash sync speed, 1/125 instead of 1/60 (later 19 has a sync on 1/125) of the 19, and a need for four batteries instead of two used by 19, the increased power requirements probably caused by the more complex electronics. The film speed setting dial has also been moved from the bottom of the camera to the top, the dial being around the film rewind wheel. Top shutter speed is 1/1000 on both cameras.
Both Zenit 19 and 18 use PX625-type batteries, which are hard to get nowadays; but fortunately both cameras use a bridge-type metering circuit, and thus alkaline or silver oxide type 625-size batteries can be used without any loss of metering accuracy, and without the need for exposure meter recalibration.
Unfortunately the electronics are unique to this camera model, and apparently only one dedicated lens allowing full-aperture exposure metering was designed and manufactured in small numbers. This is the Zenitar EM 50/1.7, which also has an odd square aperture formed from only two L-shaped parts. The automatic exposure system of course works with all M42 lenses, but requires stop-down metering to be used when using other than the dedicated lens.
The dedicated lens is even rarer than the camera itself, and the camera itself isn't very common, with only 7001 examples being made it ranks among the rarer conventionally sold Zenit cameras. The model was manufactured between 1980 and 1987.
Experiences on using the camera Edit
There are several slightly odd features to the operation of the camera. Notably, changing shutter speed requires lifting the dial before it can be turned to another position, but the film speed selection dial can be rotated without lifting, which does make changing shutter speeds slightly tedious (apparently the designers thought that the automatic exposure would mostly be used), and changing film speed by accident somewhat easier. (Added note: The lifting dial has been a trademark on all previous Zenit SLR's, so it seems that the desiners kept this function)
The shutter speed dial is also slightly small to be easy to use, especially as it must be lifted before turning. Activating the light meter is also a bit tricky, requiring the shutter release to be pressed quite hard, nearly hard enough to trip the shutter, at least on some examples. Also releasing the shutter button immediately cuts power from the electronics, or so it seems, as longer exposure times require the shutter release to be held down for the full duration of the exposure, which is otherwise cut short.
The viewfinder is bright enough to be well usable in normal lighting conditions, but the exposure meter isn't very easy to see, having black markings, and only displaying overexposure, underexposure and shutter speed of 1 s, 1/30 s and 1/1000 s on the scale, thus giving only an approximate idea of the correct shutter speed. This shortcoming is made worse by the lack of a match needle, making deciding exact choice of shutter speed somewhat harder when using manual exposure -- helping to suggest that it was originally thought that the aperture priority mode would be used most of the time. Some examples are known to have a split-image focusing aid in addition to the microprism area, but it seems that most have only the microprism area; the split-image equipped version is considered to be rarer, possibly even an after-market modification.
Generally the camera body is well built, and does have a solid feel, is tolerably easy to use once gotten used to and the auto exposure does also give at least usable results in most conditions, apparently being center-weighted. The Zenit 18 is both a usable, well-built M42 camera with some interesting features, and a quite rare, historical piece of Soviet camera development.