I've been using it for a while, and I think as @ProfessorYakkington said, it depends on what you're using it for. I use it for work, and have work pay for it. In this case, I don't need absolute privacy, I need a contractual data guarantee, and their public TOS is (more than) sufficient for what I need for basic search.

It's hard to imagine a functional business relationship in most realms where the company you're doing business with has 0 knowledge of who you are, especially on the Internet. To provide search results kind of requires "knowing", at least for a second, what you're searching for. I think Kagi has a more private model for tailoring the results than traditional search. Instead of hidden filter bubbles, Kagi has transparent "lenses" you can choose to apply or not. The most useful one to me is the "forums" one, which refocuses on actual forums for results, like technet, askubuntu etc...

Not having to fight off ads, and having a pretty obvious method for them to make money(i.e. you pay them for service) is all to the good IMHO. The results seem to be on par with StartPage, with one difference. The forums lens is better at finding "real answers" for tech questions than StartPage which often finds the same "SPAM" results Google does. This is unsurprising as StartPage is anonymized Google. This may or may not be a good thing. If you're OK with ads or ad-blocking(you should be) - why pay for Kagi when you can use StartPage for free? The main reasons are to support a different search model, to get the lenses -especially forum focused, and for their GPT like results with citations.