I currently have ~170 tabs open, which is about average for me. I can honestly say I've used the tab search twice in the past year. I use instant search every day.
As a developer, I'm always being pushed to focus on a more widely used browser, but the unbroken quick-search experience is unmatched and immensely valuable to my speed of working and browsing.
I'm gonna downgrade, and reconsider alternatives, too - all my favourite passive features in Opera are available from other browsers, along with a more comprehensive suite of extensions and comparable UX when you take out Instant Search.