Could you elaborate further on what you want to achieve?  Makes it easier to come up with an example. Thanks!

 /Kristoffer

I'm also trying to do the same as Claus.

In my case I want to get all users that has an attribute not set to String.Empty and null.
uq["MyAttribute"] != String.Empty && uq["MyAttribute"] != null

I was trying this but it did not do what I want. 

UserQuery uq = new UserQuery();
StringCriterion strCriterion = new StringCriterion();
strCriterion.WildCardType= WildCardType.None;
strCriterion.Value = String.Empty;
uq["MyAttribute"] = strCriterion;
UserCollection users = CommunitySystem.CurrentContext.DefaultSecurity.GetQueryResult(uq);

Hi guys,

 Try this:

public class NonEmptyStringCriterion : StringCriterion {
  public override string GetQuery(string propertyName)
  {
    StringBuilder queryBuilder = new StringBuilder();
    queryBuilder.AppendFormat("NOT ({0} {1} '{2}'", propertyName, EQUALS_OPERATOR, String.Empty);
    queryBuilder.AppendFormat(" {0} {1} IS NULL)", OR_OPERATOR, propertyName);
    return queryBuilder.ToString();
  }
}

Instead of assigning a StringCriterion instance to the uq["MyAttribute"] property, create an instance of NonEmptyStringCriterion and assign it instead. This will find all nonempty attributes.