I’ll cut to the chase. There is really only one good search engine for Rails applications at the moment of this writing.
It’s called ThinkingSphinx, it’s fast, it’s reliable, and it has the easiest Rails implementation, that I have seen so far.
There is really no need for me to do a tutorial here, since Pat Allan has already done a great one himself. Read it here.
I use ThinkingSphinx in production on my Gipote site. Before that I’ve been with Ferret (not good in a multi-mongrel production site), and UltraSphinx (gave me great problems when indexing multiple models). After using ThinkingSphinx as my search engine, all pain has gone.