Dead cassette too

Despite my relatively frequent chain changes, my rear cassette has died. I've put either 3 or 4 chains on the bike in 2 years at 800-1,000 mile intervals. I guess the winter riding and all the crap on the roads that sticks to the drivetrain takes it toll. I really don't think I'm a believer in the recent craze in 10-speed cassettes. It makes the chains very narrow and much less durable. There is less tolerance all around. They're fine for fair-weather training and for racing, but as a commuter, I feel pretty ill-served.

After changing the chain this last time, it's been skipping in the middle gears. I checked for a seized link and for the correct chain length. They check out fine. I measured the last chain, and it's not that bad. Not quite the 1/8 inch stretch over 12 inches that most references suggest doing a cassette change at the same time. On closer inspection of the sprockets, it's pretty clear the teeth are worn badly on the sprockets I tend to use the most. I had the mechanic at the Trek store swap out the cassette with the one on my parts bike (the one I got hit on a month after getting). While it was sunny, I went for a 20+ ride in the hills and didn't experience any skipping.