I hate to report this as not working, but it's true. I'm using Relevanssi with custom code to use checkboxes to select multiple categories to search in. Typically, you can add a hiden input field in your search function that says
<input type="hidden" name="cat" value="<?php echo $catlist; ?>" /> and the default WordPress search will search only in the categories chosen, as supplied by he $catlist variable. However, I've found that something in Revlanssi strips that default functionality, and only searches the very first category in the list - and ignores all others.
I don't mind that this plugin has a "premium" (i.e. "paid for") service that will give you advanced functionality, but I *do* mind that this plugin strips the default functionality and forces you to buy the premium version to repair it. That just doesn't seem right. I'm not implying that the author of the plugin is trying to rip anyone off (truly, I am not - this is really a great plugin) - I'm just hoping this is just some bug that hasn't been noticed yet and can be repaired, because *default* functionality shouldn't be messed with, except to improve upon it.