It is not - check out Chatter Cloud Swarm on the AppExchange for some code on how this can be done.  

I'm working with the author to refactor the existing package to make it scale better.  Here's a sample of the unpublished new code: a trigger + asynch method:

trigger opptySwarm on Opportunity (after insert, after update) {
   List<Id> opptyIds=new List<Id>();
   for (Opportunity o: trigger.new){
     opptyIds.add(o.Id);
   }//for
   
   opptySwarm.evaluateOpptyRules(opptyIds);
}

public global class opptySwarm{
    
    @future
    public static void evaluateOpptyRules(List<Id> opptyIds){
        List<Opportunity> opptys=[SELECT Id, Type, AccountId, StageName, Amount FROM Opportunity WHERE Id IN:opptyIds];
        
        String userType = Schema.SObjectType.User.getKeyPrefix();
        String groupType = Schema.SObjectType.Group.getKeyPrefix();
    
        List<Opportunity_Swarm_Rule__c> rules = [select id, type__c, Opportunity_amount__c, Opportunity_stage__c, Opportunity_type__c, user__c, group_id__c, ownerId 
                    from Opportunity_Swarm_Rule__c];
        List<EntitySubscription> subs=new List<EntitySubscription>();
        
        List<EntitySubscription> existingOpptySubs=[select SubscriberId, ParentId from EntitySubscription where ParentId in :opptys];
        Set<String> existingOpptySubsIds=new Set<String>();
        for (EntitySubscription es:existingOpptySubs){
            existingOpptySubsIds.add((String)es.SubscriberId+es.ParentId);
        }//for
        
        
        Set<Id> acctIds=new Set<Id>();
        for (Opportunity o:opptys){
            if(acctIds.contains(o.AccountId)==FALSE){
                acctIds.add(o.AccountId);
            }//if
        }//for
    
        List<Account> accts=[SELECT ownerId, Id FROM Account WHERE Id IN :acctIds];
        Map<Id,Id> acctOwnerIds=new Map<Id, Id>();
        for (Account a: accts){
            acctOwnerIds.put(a.Id, a.ownerId);
        }//for
    //logic - for all opportunities, 
    //check if any     
        for (Opportunity thisOppty : opptys) {
        
            for (Opportunity_Swarm_Rule__c rule: rules) {
    
                if (rule.Type__c.equals('All Opportunities') ||
                    (rule.Type__c.equals('Only Opportunities for Accounts I Own') && thisOppty.AccountId != null &&
                        rule.OwnerId == acctOwnerIds.get(thisOppty.AccountId)) ||
                    (rule.Type__c.equals('Only Opportunities above a certain amount') && 
                        rule.Opportunity_Amount__c <= (thisOppty.Amount)) ||
                    (rule.Type__c.equals('Only Opportunities that reach a certain stage in the sales process') && 
                        rule.Opportunity_Stage__c.equals(thisOppty.StageName)) ||
                    (rule.Type__c.equals('Only Opportunities of a certain Type') && 
                        rule.Opportunity_Type__c.equals(thisOppty.Type))                
                    ) {
                    
                    if ( existingOpptySubsIds.contains((string)rule.User__c+thisOppty.Id)==FALSE){
                        subs.add(new EntitySubscription(parentId=thisOppty.id, SubscriberId=rule.User__c));
                    }//if 2
                }//if 1
            }//for 2  rules
        }//for 1 oppty's
        try{
            insert subs;
        }catch (DMLException e){
            system.debug('Oppty Swarm subscriptions were not all inserted successfully.  Error: '+e);
        }//catch
        
    }//evaluateOpptyRules

}//class