Kummer,
eSearch is not like QueryBuilder in the fact that, like you said it does not allow the user to build a query using a GUI. The eSearch Widget does allow for open SQL statement where the user Manually types the whole where clause, including field names and operators. There is not another widget out there that is actively supported that has the features of QueryBuilder widget. The market is very small for a widget that does what QueryBuilder does, so I have not plans for adding this sort of capabilities. The focus of RIA applications is for the user to have little, to no knowledge of GIS, so a widget that is developed for GIS savvy people is not the greatest fit for RIA development.