Before class balancing, the Demoman was the most powerful class, primarily due to the
detpack. Before
Team Fortress 2.4, the detpack had a very large effect radius. Placing it strategically could wipe out the entire enemy team and rack up the
frags. Prime locations included the bridge in 2fortX and the center of the enemy prison yard in
The Rock. Of course, if
team kills was enabled, your teammates would become very irate.
In successive versions, the radius was decreased dramatically, and by the time Team Fortress 2.5 came, it was practically useless. The only real use for it would be to blast open certain objectives, such as tunnels in The Rock and The Well, and the bank vault in Quake Town.
The MIRV grenades, although not exclusive to the class, are especially powerful in the hands of the demoman. Due to the demoman's speed, it is easy to run up to the enemy's respawn chambers, toss in a few MIRV's when the chamber door opened, and wait for enemies to respawn into certain death. Another cheap alternative is to toss in pipe bombs and detonate them when the respawn sound is heard.
The pipebombs are also useful for pipebomb jumping. Much like rocketjumps, the force of the blast combined with an initial upward motion would propel the demoman to superhuman heights. Due to the increased armor of the demoman, the blast does not make too much damage. With a lot of practice (or an alias to a macro), it's as easy to pull off as a rocketjump.
The Team Fortress Classic demoman is almost the same. With medium armor and full health, a pipejump with four pipebombs can propel the demoman almost as far as a single concussion grenade. Any more and you will toast yourself. Combat is best near walls and tunnels where you can bounce pipebombs off and enemies have no where to avoid them. In extremely close quarters you will have to use a shotgun or crowbar. Open spaces are more difficult as enemies have lots of place to avoid your projectiles, but with good timing and anticipation a skilled demo can do just as well.
The demoman is an excellent defensive class as the flag can be trapped with pipebombs. Two demomen working together can keep the flag practically still with proper timing. On attack/defend maps like dustbowl, a two-demo cycle can effectively seal a chokepoint. On offense, the demo is not quite as useful as trapping-and-waiting isn't meant for offense. However, as an assistant, can propel teammates long distances with little to no damage (depending on friendly fire settings) and trap for enemy chasers. On attack/defend maps, demos are useful for protecting grenade packs and retaining gained ground, sometimes more effective than a sentry gun.