Did a lot of work on the erg during a long term injury spell 4-5 years ago.
Agree with most of the previous respondents in that it shouldn't really be looked at as a replacement for quality running workouts. It could possibly be used instead of a traditional weightlifting session or as additional aerobic work on top of a specific running program. A 500m time trial done properly with leave your whole body absolutely jelly for a while, and you'll possibly be in need of a bucket if you've eaten recently.
Looking back at old training diaries, when I was doing it I got to 1:28.2 for 500m, 6:46.5 for 2000m, 17:48.9 for 5000m, 37:30.6 for 10,000m, 1:21.24.8 for 21,097m, and 15,960m in a 60 min session. Hit a top speed of 1:17/500m during a 30sec all-out sprint. Very sore sgoulders, arms, and gluteus maximus muscles after long sessions on it, so they weren't very frequent! Combined it with stationary bike, stairmaster, and pool running workouts for variety during that time. Haven't used it for a long time now though.