The page for enabling beta features directs me for information to https://www.mediawiki.org/wiki/Search. There I do not find a short, understandable description what the new search features are at present and what the difference to the old search is. Therefore, I do not bother to test this feature.
Topic on Talk:Search/Old
I took a stab at adding that: https://www.mediawiki.org/w/index.php?title=Search&diff=1180052&oldid=1173154
How should a beta tester know what may be a bug and what not, if there is no specification of what is to be expected?
The old search system didn't have a spec at all. Cirrus has grown one in the form of somewhat readable tests (http://git.wikimedia.org/tree/mediawiki%2Fextensions%2FCirrusSearch.git/master/tests%2Fbrowser%2Ffeatures) but if you know something used to work and it doesn't or if you think the results are worse then that is a good time to file a bug with examples.