Well, I used to have an earlier version installed, but I hardly measured my system productivity. I found the search very ineffective, and later after reinstalling my xp system I never installed it again.
I've noticed that Microsoft has been doing very poor job on add-ons and power toys. Although Vista's new search engine is very good.
As for XP, I prefer and recommend ViStart project, which I'm sure you know of, it
instantly searches for programs and documents. Not exactly as Windows Search, but works fast and actually finds files. And ViStart runs only if you launch it, no background services. Looks fine with Zune style.
The native XP's search is not good either.

Just my opinion.
P.S.
Forgot to say about
Copernic Desktop Search, it is similar to Windows Search, not good either (L O L).