The make up of the EMT 156 Compressor isn't too hard to follow as the parameters are found on most compressors. You've got a variety of parameters including ratio and gain, as well as a slew of other buttons and other knobs that you have to work with here. While it would take too long to list all of the parameters available on the 156, suffice it to say that there is more than enough to work with here, making it a versatile compressor. I haven't seen the manual for this compressor so I can't really say anything about it...
The sound of the EMT 156 Compressor is absolutely top notch. While EMT isn't known for making dynamic processors like compressors, this is truly a great sounding piece of gear. When I usually think of EMT I think of their plate reverbs and not necessarily this end of their products, but this is overall a very impressive sounding compressor. It is great for a variety of applications since it has so many different parameters and buttons to play with. I've had a lot of success using this on vocals, guitars, drums, and electric pianos. I don't think I can think of an application where I wouldn't be comfortable using this, as it is simply an overall great sounding compressor.
While I can't say I would recommend seeking out an EMT 156 Compressor as they are hard to find, I would definitely recommend looking into this compressor if you are interested in getting a classic compressor with top notch sound quality regardless of the price. I've only seen this compressor in the one studio that I used it at and have yet to see one at all since using it. I love how many different buttons and knobs it has, and even though there is a lot going on it definitely doesn't have a convoluted interface. All in all, the EMT 156 Compressor is a top tier piece of outboard gear that I would recommend without a doubt - if you can find one!