In general Relevanssi does work. So, when something doesn’t work – you can’t search for custom field content, results look bad, and so on – that means there’s a problem.
If you updated to Relevanssi free 3.0 or 3.0.1 and the plugin doesn’t work anymore, you can probably fix the problem by deactivating and reactivating the plugin.
If it’s not that, then approximately 90% of the time the problem is the same: there’s a
query_posts() call on the search results page.
Any calls to query_posts() on search results template make Relevanssi fail silently. You’re not getting results from Relevanssi any more. This is unpleasant, but something I can’t really fix, considering that WordPress doesn’t offer any official way to replace search results and the way Relevanssi is coupled with WordPress is something of a hack. So, something simple like this can throw Relevanssi off.
If you’re having problems with Relevanssi, the first step is to check your search results page, usually search.php. See if there’s a call to query_posts() and if there is, comment it out. In most cases that is enough to fix the problem.