Hi there.
We are using JA K2 Search module in a project of ours. Though it is having great core functionality it lacks any advanced interface options. Some of these are:
– jQuery/AJAX loading of search criteria (search fields) + normalization. When you choose an option from one search criteria, all other reload, or their options reload (thats why we need AJAX) and all options that have no result hide. For example if you have two custom fields vegetables and color and when you choose to search vegetables with green color, in the other group the option “tomato” hides, because there is no result for green tomato.
functionality like that, for VM can be seen here: http://webinteractions.ru/en/demo
– Possibility to add AND remove criteria to/from the filter and see result. It can be seen here for VM: http://www.galt.md/index.php?option=com_virtuemart&page=shop.browse&Itemid=1&category_id=22&limitstart=0&product_type_id=5&product_type_5_brand_comp=in&product_type_5_brand[]=Apple&product_type_5_brand[]=Asus
This, I think, can improve greatly the usability of the module. Are you interested to work in that direction?