I tried Neeva and immediately hated it - it felt slow and clunky. I tried Kagi and immediately loved it enough that I've been paying ever since they let me. I think the big thing that Kagi is doing differently to Neeva is just being good.
I know right! I can't remember the last time I had to click through to the second page of Kagi unless my search was a futile one in the first place (ie: I hadn't understood the problem enough to know what to search for)