I currently have 9/80's. It's an awesome perk and holds pretty significant value as a benefit. It's pretty huge to load up your weekend chores for every other Friday and no longer have that crap consume your weekends. For example, on my Friday's off I wake up, get my work out in, wash my wife and my cars, then do stuff around the house. Last 9/80 I power washed the driveway, patio, and house. This 9/80 I'm going to re-grout and seal a bathroom.
To the complication of 9/80's, they can be as complicated or simple as you make them. They were fairly complicated in the past at my company. To the point they put them on hold, reassessed, did manager training, and re-implemented them.
They're pretty straight forward now. Correct, not every business unit can have them, kind of is what it is. Those that can, it's up to the manager to allow the benefit and to manage it. You can only use them on Fridays, no swapping for Mondays, Thursdays, etc. It's much easier to manage just Friday, when the office is generally slower anyways. Becomes a lot more of a pain to the rest of the company when you can't hold meetings on Mondays, etc. Also more difficult to manage coverage when days are constantly changing.
If your scheduled 9/80 falls on a holiday, for example Thanksgiving, you basically just lose it. No, you can't take Wednesday off. See above, just adds to the complication.
The manager splits the 9/80's evenly. So half my team is in every Friday. I have a counterpart I keep up to speed on any projects/questions that may come up. You also have to be available on your 9/80. I don't stay logged on my computer but I have my phone with me and access to my computer so I can log on and answer any questions quickly. If you're going out of the country, won't have access to internet, etc., you have to take a vacation day, you can't just use your 9/80.
If you have a meeting, project, etc., you have to come into the office. It may just be for an hour, but come in for the meeting. We aren't giving other business segments reason to gripe about our 9/80's.
If your 9/80's are mismanaged, you aren't responsive, etc., your team loses them. It's as simple as that.
As for the employees, we all greatly value them so will do whatever we need to keep them, i.e. everyone on my team goes above and beyond to answer questions in a timely manner (w/in minutes) when they are off on a 9/80. For the business units we work with that don't have 9/80's, we don't really hear or see any grief from them anymore. They know they can still count on us to get our work done. They also get to the office before me and leave after me.