My son has taken one from 3rd to 12th level. Sometimes the class is not that effective in combat compared to a fighter or barbarian (particularly against large groups of smaller critters). Sometimes it's ridiculous. The key seems to be that you can use smite after you make your attack roll. Get advantage to get more chances at crits. When you score a crit, use a smite.
A 1d8 weapon goes from 5-13 damage to 29-36 damage (using our modified crit rules) with a single second level slot. He's using a weapon that does 1d8 bludgeoning plus 1d8 thunder damage and has three 3rd level slots. He managed to use a third level slot and a second level slot on crits in the same fight a few sessions ago. That brings a fight to an end very quickly.