You don't know what you're missing if you don't have this installed; the autocomplete search is EXCELLENT. The first 5 people I showed it to were all genuinely excited. My co-admin commented "This is so nice. The default XF search is somewhat clunky, this feels very modern and nice."
The similar thread suggestion feature I feel is better than anything else available for XF. I have it visible below Quick Reply and when typing a title for a new thread.
It reduces the number of duplicate threads and is one of those things that helps any forum.
Sure, this add-on requires ElasticSearch and XF Enhanced Search, but I wholeheartedly recommend even smaller forums getting them so that you can take advantage of ElasticSearch Essentials, its autocomplete search and similar thread suggestions.
As for how Xon supports his add-ons, it's out of this world. Had a small bug which I reported on a Saturday night, and wow, Xon debugged it in incredible detail and provided a fix right there and then.
If only all developers were like Xon...!