Probably a nice framework but if you're not using SLF4J you're toast when it also has a backdoor. SLF4J does not have an issue. Swapping out log4j for a different logging framework is a day work for all my applications at work. Not that we where using log4j we where using logback that's the current industry standard anyway.
I can't imagine what you'd be doing with your logging where swapping logging library wouldn't be almost entirely done with a single digit number of sed commands.
I'm a huge fan of SLF4J. We happen to use logback with it and a custom appender that send messages over ActiveMQ. We regularly take log volumes of thousands of messages /s on modest hardware into a Graylog instance.