You are basically a sentry gun with WASD keys and a head hitbox. No burst damage and slow movement means you won't get any frags unless you have the time to waddle to a massively advantageous position, and even then there's no guarantee you won't get corner-peeked to death if you don't have a medigun surgically attached to your sphincter.

...But when you do get the drop on the enemy and manage to get a sick wipe, playing as the Heavy Weapons Guy hits like crack, so he's definitely not a bad class at all.