|Dire rat (source)|
My players got a little crazy with this one. The players of the fighter and warden are actually engaged. When they were planning who would stand where, the fighter asked the warden to stand in a specific place, but he didn't. The fighter (who was already infected), got pissed off and actually bit the warden, thus infecting him as well. He was pissed! I guess this may be why some people dislike running games involving couples, but that for another topic.
During their next extended rest, the fighter, barbarian and warden managed to heal themselves of the disease. The ranger, with his limited endurance, did not. He was dismayed to find that the town nearby was not large enough to have a doctor and insisted they find a city large enough for him to be healed. Luckily, they were going that way anyhow. In the 4 days it took them to walk to the city, he's condition actually worsened, so I guess it was worth it (despite the barbarian getting them throw out of the doctor's shop).
So clearly, including a disease in my game added a lot more than just making things a bit more difficult for the players. It added some roleplaying (which my table tends to lack - they like hitting stuff) and influenced their decisions on where to go and what to do next.
While I certainly don't want to overuse this mechanic, I feel like it definitely has some cool features and I will be including more diseases in future encounters/campaigns.