I was looking for an autocomplete for an addressbook kind of functionality. The requirement was that users can have FirstName,LastName,Email,Type and when user starts typing they need to be searched first by LastName,FirstName,Email and some types were to be given higher priority than others. Doing this on server side means too many calls to be sent to the server so doesnt seems like a perfect solution. Also mostly users will have <100-1000 contacts in addressbook so why not do it on client side. Looked into some autocomplete plugins but none of them were what I wanted and finally landed onto this http://code.flickr.com/blog/2009/03/18/building-fast-client-side-searches/
I have never found such a perfect solution to a complex problem before. Thanks flickr for sharing it ;).