My Gen 4 has 18,000 rounds since July of '11.
Within the first 900 rounds I had 12 stovepipes and 20% of brass to the face, left, right, up, etc.
Installed and aftermarket extractor, guide rod, extractor depressor and 13 lb. recoil spring and got 3 stovepipes in 2,100 rounds or so, and 10% brass to the face, left, right, etc.
Sent the gun back to Glock, and got a new extractor and ejector.
Since the trip to Glock, I have about 15K through it and I've only recently started receiving a few cases to the forehead (1 in 300) in the last 2K rounds, but no more FTEs so far.