there are ways to search information in userfields without having to dedicate a userfield to any one (singular) purpose.
consider this site:
http://www.hgltd1.com/
the links on the left are the categories with products normally entered in the dB. the links like gold, silver, diamond and the like are these same products that are tagged with those keywords in a userfield. some products have maybe 5 or 6 keywords in a single userfield. when linked to properly only the desired keyword will display the product regardless of the number of keywords in the userfield.
in some instances userfields can be multifunctional so don't squander your precious userfields by dedicating to one function if it can be avoided.
dan