simple CFC question

You could make your returntype a string. But Craig's answer is better.

To expand a bit on what Simon mentioned, here's how you might structure the CFC Method/Function to return a boolean:
<cffunction name="updateUser" access="public" returntype="boolean">
<cfargument name="userId" type="numeric" required="true"/>
<cfargument name="username" type="string" required="true"/>
<cfargument name="email" type="string" required="true"/>
<cfset updateOk = true />
<cftry>
<cfquery datasource="dsn" name="qryUpdateUser">
update user set username='#arguments.username#' where userId = #arguments.userId#
</cfquery>
<cfcatch type="database">
<cfset updateOk = false />
</cfcatch>
</cftry>
<cfreturn updateOk/>
</cffunction>

If the query above runs without an error, the value of updateOk remains true. If there is an error in the DB, the CFCATCH block will be run, setting the value of updateOk to false.

At the end of the function, the value of updateOk is returned to the calling page/code block.

Then, with the code Simon provided, you can use this in your calling page:
<cfif updateUser(userID, name,. email)>
Success
<cfelse>
Failure
</cfif>

Wasn't sure if you were just having trouble on the calling page (where you want to output the success/failure message or with how to return the boolean value from the function as well. Hope this helps.

The return type describes what the function returns. So with a return type of boolean you can only return a true or false (or something that cf can convert to a boolean).

So you can do:-

<cfif updateUser(userID, name,. email)>
Success
<cfelse>
Failure
</cfif>