Well many comparisons can be drawn, but you have to consider certain factors in when comparing a consumer product from the late 1960's to modern day professional cine lenses.
For what the Iscorama is:
By itself, its a gem of an anamorphic lens, no question. It's tack sharp, good edges, great optics and saturation, beautiful flares (if you have a single coated version), an of course, out of all the old lenses, the best patented focus system..
For what the Iscorama is not :
It was made in 1968 as a high end projection attachment, so in part it's really not a stand-alone lens, it is an attachment. We keep comparing it to actual lenses. The housing of the lens in complete plastic, is to say the least, it's very dangerous and doesn't protect the optics at all, and if the lens was to encounter any kind of harsh weather conditions etc. it would completely fail. And as far as the focus goes, although one of the best for what it is, it's still a projection lens so close focus becomes somewhat of a process that you just would not encounter on any proper cine lens. And ultimately, because it is still an attachment, you will run into only being able to use a set amount of focal length primes, which depending on the cinematographer or certain shot, is a no go..
In General Comparison :
The Isco has GREAT optics. I've managed to pull images from that lens that look better than some of the most expensive lenses I've ever used. But like I said above, price tag means nothing. Great example of that is the helios 58mms. They are one of the sharpest lens I've ever used, and dirt cheap. The big expensive lenses are kind of like buying a rare sports car. Beautiful to look at as far as engineering and build, but more or less afraid to use it. Afraid to "take it out of the garage" and put millage on it if you will.. Personally, when I'm using Arri's or Cooke's, anything thats over 50k in glass, I get a $1,000,000 issuance policy when shooting with them. Too risky if not, if so much as a nat lands on the lens, I start to worry, so thats my personal way around dealing with stress of rentals and or price tags. BE SMART!
I think for what most of us are here to do : Get the best image possible with tools we've researched and perfected, but for a personally attainable price.
It is a fantastic option, and no doubt ONE of the best, if not THE best, for the price. I currently don't have 40K to throw down on a set of OCT-19 square or spherical anamorphic sets, but if I did I would go that route. They are built like tanks, and the optics have so much character. BUT they are big, heavy, and expensive cine lenses. They are not fit for lightweight run-and-gun situations. I still to this day see many top notch lens companies copying the Lomo system for anamorphics, so that usually means they we doing something right if 50 years later the top dogs or still trying to produce their design and look.
At the end of the day there are so many factors that go into getting "a good image". A lens ultimately is just a tool to help achieve the look you are going for, nothing more. Storytelling, lighting, good characters, and proper operation of the camera are the KEY components of filmmaking. I don't care how much your lens costs, or how rare it is... If you just make another cool-looking vimeo lens test, your not a filmmaker, nobody really cares. No story=no substance
You could shoot a short film with your iphone about a bird you found on the street that is dying, and if it's emotional enough and people care about that bird at the end of your video, then it's good. No lens can do that. Thats filmmaking.