I've not been so impressed by a search engine since Google hit the waves.

http://www.cuil.com

What is particularly impressive is the usability. And the category explorer that so far seems to be mostly spot on and very useful. I find myself going to it more than the search results, it doubles as a kind of mimetic thesaurus.

I have seen it get confused.

It does seem to be suffering from its sudden popularity.

I agree with almost all the criticism of it, especially the lack or low ranking of Wikipedia entries.

http://blogs.siliconvalley.com/gmsv/2008/07/size-may-matter-but-theres-more-than-that-to-achieving-satisfaction.html

http://www.readwriteweb.com/archives/cuil_good_but_not_good_enough.php

http://www.makeyougohmm.com/20080728/5239/

http://searchengineland.com/080728-000100.php

Despite its nice UI/UX, in the end, the true user experience depends on its usefulness. Does it function as a good search engine?.... and to that my answer is an emphatic 'almost'.

I'm not going to switch from Google but its the best usurper I've seen yet.

*Update*

Ive started using Cuil as a research tool, not for its search results, but for the "Explore By Category" box.  When its correct, its associations are educational. Almost a Wikipedia entry generator.

Mostly its correct or humorous, sometimes both. eg. Mark Pesce is listed under "Psychedelic Advocates And Proponents" and "Psychedelic Researchers" the rest are spot on.