Now that the Engineer update has come, there will be lots of Engineers building up everywhere. How should this best be handled?
Lots of medics with lots of ubers on high-damage-dealing classes. Demomen, if they are any good, usually work best against a sentry farm — stickies are perfect for this job, the usual problem is living long enough to deploy and detonate enough of them.
Heavies or pyros can also be effective, if they can get close enough (short range for a heavy, point-blank for a pyro) without being overwhelmed by the knockback before they can do damage. The medic should lead the uber combo in (particularly if the partner is a heavy) — he’s faster, so he can absorb the knockback better.
If at all possible, back the uber team(s) up with as much raw firepower as you can muster — the aim is to either overwhelm the engies’ ability to repair stuff, take out the engies directly, or take out dispensers. Soldiers are good for this support role, as they can place well-aimed damage from behind the uber. Demos can support well, too — their indirect fire can damage engies and dispensers without having to kill the sentry first.
Sending a spy in to sap just before or during the uber(s) can be a good idea, too.