The Cooke S5's have been out for many years now, but for some reason never really took off. One of the contributing factors was that the powerhouse camera rental place Otto Nemenz in Hollywood never bought a huge inventory (as thy were knee deep in the encouragement and development of the Leica film primes). This kind of "hurt" the S5i's and they really never got the widespread use and recognition the S4's did. They never became that lens that everybody wanted to rent (like the Leica's, MP's and S4's did when they were introduced). So, many cinematographers never developed a relationship with the S5's.
I was the same, used them sparingly myself, maybe once or twice when they were new, but that was about it. My reconnection was on a job abroad with limited lens availability just last year, there were no other "good" fast lenses to rent and I thought I'd try the S5's again. And although I do find the series lacking in focal lengths (today, it's imperative to have something in the 20-22mm and 27-29mm range, in my opinion) and that they are way too big (Master Prime big, which is a huge problem in this day and age when everything is handheld), they do really produce excellent images. On a job I'm doing in Toronto now, I combined them with some very mild GlimmerGlass 1 diffusion and am really happy with how they look. They have that organic feel of Cooke S4's we've all come to love, but are faster and have that sense of falloff that I never feel with the Leica Summiluxes, despite them both being the same aperture.
So I encourage DP's to maybe "reconnect" with them and try them on a little job again. You might find that they're easy to love. They've become my go-to set of fast lenses these days.