When a demo joins the server I'm in, I leave the server.
I really dislike the demo class.
On Suicidal, Demos are frequently told to change class or get kicked. It usually ends with a kicking. Past Hard (And one could even argue that it's really normal where they shine) their damage is pretty sad, disrupts the team with screen-shaking, obscures vision with smoke, and pisses off Scrakes/FPs that they tend to main but never actually kill.
Dead-serious, you'll have a hard time taking on anything bigger than Clots and crawlers with the 'Nade launchers on Suicidal. 2 DIRECT HITS from a Level 5 demo to take down a freaking Gorefast, taking almost double that if it's only splash. Plus, until they're Level 5/6 (And few are. Most that are have farmed...) their gear is immensely expensive, so they're a money-sink for the team.
On Normal/Hard, they shine taking out FPs quickly and decimating hordes. On Suicidal, a Pipe Bomb and 6 Nades STILL doesn't down an FP and nades only scatter the horde out and potentially ruin teammates shots while simultaneously covering the enemy in smoke. Probably the worst factor to this is that good demos are VERY, VERY rare. Many, many Level 6'ers have farmed somehow, so it's not a representation of skill in most cases. And nothing is worse for a team than a bad demoman.
And the insane pipe-bomb spam by Level 6 demomen is not only a money sink as a trio of Clots waste his pipes or a Scrake/FP charges over them, detonating them without a scratch, but it steals kills from other team-members while obscuring vision. IMO, the massive no-skill minefields of Level 5 and 6 demos is the mark of a nublet, not a skilled demoman. The only thing it's good for is making me laugh as they gib themselves on their own pipes.
A team with more than 1 demo is usually unplayable due to "Boom spam". I tend to switch servers if there are 2 or more demos. And the excessive pipe-bomb-spam that high-level demos put out is nothing short of ridiculousness. The class needs a look by Tripwire, because as is, it's more of a nuisance than a help to a Suicidal team.
Sorry if this is coming off as harsh, but I'm really, really sick of Demomen wrecking otherwise-good suicidal teams. At the current time, demoman isn't a viable suicidal class.