Sermon based small groups are a great way to link a weekly sermon and church service with a small group.  The small group will study the same Bible verses and topic that was covered in the sermon, giving people a chance to go look deeper at the topic and actively apply it to their own lives. 

These groups have a built in, directed topic to cover which can make conversation flow easier.  Also group members who may have missed the sermon can get caught up on what the church is covering. 

These small groups do have challenges, however.  Mainly they need a leader who can keep the group on track so it does not just turn into a critique of the sermon.