We know that some of them have more internal employees and more bandwidth then we do.  We know that some have them have lower pricing then us.  Some of our competitors have very strong, long standing relationships with clients.  Some are more niche so they can fill very specific jobs faster and they have a pool of qualified people ready to go and can be more proactive in a particular area. 
As far as weaknesses go, they do not do as much due dilliegence as we do.  They do not check references, they do not go out and meet with the clients in many cases.  They just send anyone and see if they stick. They don't really care about finding the right fit, they care about filling orders as fast as they can.  Some of them do not have experienced talent working for them. They have inexperienced recruiters who are not as thorough and commited as we are. 