How many used Marvel Classic (or some variant like FASERIP or G-core) for settings aside from the Marvel Universe(s)? Why or why not? Does it work well for setting with different assumptions and 'physics' than Marvel?

I have used the system for a DC only game..

i have used it for a spy game (everyone was a secret agent-james bond type character, originally we were going to use the top secret game system, but some people had issues with it so we went with marvel, rolled everyone up as human and instead of powers they just got extra talents or equipment (Q-branch type stuff) - which simulated certain powers. (laser watch, re-breather, car that could also fly or drive underwater).

I'm actually in the process of preparing to use the classic marvel rules for a Dresden Files campaign.
Dresden Files is a series of 15 or 16 books that can probably be described as urban fantasy. It follows a wizard who hires himself out as a private investigator. If you're looking for something engaging to read, I'd recommend the series. But it doesn't start getting good until the third or fourth book. Then it gets amazing.

The Dresden Files role playing game uses the Fate game dynamic. But those rules are extremely narrative and story driven, without a lot of rules or crunch. You can be a sorceror or a wizard type, pure mortal, vampire, werewolf or a host of other backgrounds. But the problem is it doesn't give you any direction on what powers you can have. So I've taken the very few powers listed in that rulebook, added the 1500 powers from ultimate powers, plus cantrips and divine blessings from AD&D.. and now we've got an incredibly diverse mix of character choices.

So we've borrower character creation from this ruleset, the powers, feat rolls, and I think it's going to turn into a fun campaign when we start in the Spring.

Oh lordy I've used it for all kinds of stuff, including a Grand Theft Auto-esque heist game.

We have used Marvel RPG for a multitude of game settings. As Dark said, it is great for Spy scenarios. Our current offline campaign is human agents of the Illuminati, with minor mystical abilities. (Very low level regeneration - so useful in keeping characters in play, fire suppression) as well as (as Dark said) Q branch equipment (disguises etc)

I have been using the Marvel system, or FASERIP since i bought the original set back in 1984, have kinda home-brewed up the rules a bit by incorporating some things that i found in other systems and whatnot, but all in all it's worked for me and my players over the years.
I have never used straight Marvel characters, instead my best friend and i just created our own world and our own characters and just went hog wild over the years. We used the SAME world each time we played and it became one giant ongoing and long-running creation...characters/players came and went, characters died and/or changed their powers, skills, identities, and we made sure to keep track of what went on in our world so as to recap for any new people that came into our world to play.
It went smoothly for years...i mean decades, until around 2015 things came to ahead, the crisis' became more and more intense and i decided to restart the entire world/timeline over and create a new world with some hints and shadows of the world before so as to not start with a clear clean slate.
So now in 2020, my kids have grown up and have characters in this world and the last one, i have a stable group of players that are really invested in their characters and the world they live in and it's old school: we gather in my front room, grab pen and paper, roll our dice, and create magic!
My games are part RPG and part theater, we get into it, we have all become actors living out our superhero fantasies in the comfort of my living room....we don't dress up or anything like that (not that that's a bad thing mind you), but we really get to become our characters in our speech, our mannerisms, and our mindsets.
We have has some EPIC games over the years, we have cried when characters died, cheered when they are reborn, became angry when the villains won, and celebrated when the heroes came out on top.
I wouldn't trade any of the games we've played over the years for anything nor the players and their characters that were created.