What is an effective way to use the Sandman?

I use it a lot for things like

• Stunning an engineer who is hammering on a turret, usually while we have an uber going to take it out.
• Firing it into groups of people during a big firefight and helping my team by stunning one of the players in the fight
• Smacking heavies who are shooting with it, then running up on them and popping shots into them.
• Stunning anyone guarding a flag that I want to pick up

There are more uses than that but, that is my main usage of it. I've found it pretty useful in certain circumstances.

When fighting alone, the Sandman is a wash at best, because the small window of opportunity you get from the stun is offset by your reduced survival rate due to the health drop.

The Sandman is much more effective when used as part of a team push, or even only sniper cover, as stunned foes make excellent targets for Rockets, Grenades, or Headshots.

Target also matters -- If there's an enemy heavy parked on a control point, he makes a much better target (both in terms of ease of hitting and in terms of net effect) than a scout or spy.

If you primarily play the scout as a harasser, you may also find more use for the sandman, because getting stunned is, if nothing else, quite annoying to the recipient.