We've done lots of testing to make sure MariaDB works as seamlessly as
possible for those moving from MySQL to MariaDB. As with any upgrade, I
would definitely recommend making a backup and testing MariaDB on your
end to ensure everything works with your applications and use-cases
before putting it into production.

My company is developing an application atop MariaDB at the moment and it's been a seamless process moving from MySQL.

For compatibility purposes, XtraDB identifies itself to the server as
InnoDB, so there's no need to change ENGINE= in any SQL files. Any
existing InnoDB tables will start using XtraDB automatically after the
upgrade.

If you try to specify ENGINE=XtraDB you will see a failure because of what Daniel said... Again, we're developing against "XtraDB" and haven't experienced a problem working under the assumption that it is "InnoDB."

Yes. All of our testing indicates you can upgrade from MySQL to MariaDB
and all of your schemas, tables, data, relations, comments, indexes,
and so on will "just work" like they did with MySQL without you having
to change anything. Of course, as with any upgrade, I would recommend
testing on a non-production server before putting MariaDB into
production.

Not that this is recommended but some of our developers work against MySQL while others work against MariaDB. We're trying to transition to all MariaDB but I figured I'd say it to assuage possibly anguish others might have with such an approach for major version matching (namely 5.1 ish).

Hopefully that's helpful to you Enrique. Buenas suerte!