I have about 10,000 on a set of Metzlers and they're ready to be changed. I had Avons before the Metz and got about 8000 out of them.
I think your mileage has a lot to do with the roads you ride, your speed, and the temperature. The less I ride in the summer here, the more miles I can get out of a set of tires. I tend to run a pound or two above the suggested pressures.
I will most likely replace the Metzlers with another set of Avons. For me its much more about feel and handling than mileage. The Avons performed much better than the Metz for me. If I was worried about tire costs I would not have purchased an 800 lb touring bike that makes over 140 ft lbs of torque.
From all the reports I've read it seems there is a difference in mileage between the two brands depending on whether your on an R3T or a Standard model. Most get better mileage from Avons in the Standard sizes but from what I've read and experienced, the Metz get better mileage in the R3T size.
If you're concerned about an upcoming trip, I'd say replace them. I had a similar experience with the Avons. Taking a short 2000 mile trip I thought I had plenty of tread left. By the time I got home, they were toast. They seem to wear very fast at the end. Better to change them 1000 miles too soon than 100 miles too late. Also gives you one less thing to be concerned about on your trip.
As to the darkside suggestions. Its like religion, you're either a believer or your not. Personally I'm not.