Faithfulness is highly valued.  Aristarchus’ parents obviously had high hopes for their son because his name means ‘high ruler’ or ‘high prince’.  He was born in Thessalonica, believed the gospel, and  then travelled with Paul as one of his assistants, and faithfully shared in his suffering - including being arrested in Ephesus  (Acts 19:29; Acts 20:4).  He accompanied Paul on his journey, under Roman guard, to Rome (Acts 27:2).  From the same prison as Paul he now sent greetings to the church in Colossae and to Philemon (Philemon 1:24).  

On the other hand, John Mark (often just called Mark) had initially not been faithful in ministry.  Travelling with Paul on his first missionary journey, he turned back and left the apostolic group (Acts 13:13).  However, his cousin Barnabas encouraged him to serve according to his gifts (Acts 15:37-39).  Later Mark also became a helper to Paul in prison (2 Timothy 4:11), as well as being the writer of Mark's Gospel.
 
Along with Jesus Justus, Mark and Aristarchus both proved to be true to the gospel and helpful to the Apostle.  They were the only Jews close to Paul.  They were faithful to the gospel call and helped Paul.  At a time when Jews were being squeezed out of Roman society, they had found a home in Christ.  We do not know what they did or how they proved to be useful and faithful in ministry, but Paul commends them as examples: it would also have encouraged the minority of Jewish Christians in the Colossian church.
 
A reputation for faithfulness is very precious.  It means that someone's service has been effective and selfless: they have continued under pressure and persevered through difficulty.  It does not require great gifts but demands great commitment.  Faithfulness is often so quiet that nobody notices the steely resolve to keep going, which comes when the Lord provides the strength and love to keep serving, without obvious reward and often at considerable personal cost.  Faithfulness comes from a heart that is quietly determined to remain true to what it believes.  Believers like that rarely get a mention, but when they do it is well deserved (3 John 1:3).  However, they do not serve for the sake of their own glory, but for the glory of Jesus Christ.  And that is the secret: they put Him first before everything else, including themselves.  Faithfulness is not a medal, but the core of a character committed to serving Jesus Christ.  Are you faithful?