An out of office message is basically a quality of life feature both for you and your business contacts. By warning your clients that you won’t be able to reply to their message right away you save the time they might have wasted on follow ups. You also won’t lose prospects who might’ve believed that you’re ignoring their request otherwise.
Given free rein, I’d absolutely love to tell people that needing me to show them how to do X in Excel is actually not a vacation-interrupting emergency and there are tons of free videos that would explain that, if they did not want to contact the actual departments who handle tech support and training. Or that this project they’ve known about for a month but decided to keep under their hat until it became an emergency is something they’ll need to resolve themselves. But that would not fly at all.

But let’s talk out-of-office messages: overshares, excessive detail, the ones that self-aggrandize (I once had a coworker whose auto-replies often said he’d be in late because he “pulled an all-nighter” on various work projects, etc.), the ones that never get turned off, people who don’t use them at all, and other pet peeves.
It might feel strange to include happy client testimonials in your out-of-office responder, but doing so is easier than you think. A quick note that suggests they read the testimonials if they’re interested in a product or service you offer is an easy inclusion or postscript to your email. Just limit the amount you share to one or two.