I think the old phrase "you get what you pay for" really applies. The wife and I tore up a middle of the road ($1800) treadmill in about three years. She's only about 130 lbs but she runs on it a lot and she runs hard, I'm 6'2" 230 lbs, I only run 1-2 times a week, but I'm pretty hard on it.
Last year we bit the bullet and bought a true 500 series, and it is awesome. Quiet, solid, and reliable. We've both logged some miles on it and that thing is just like new.
If you're looking for a good aerobic workout, I think the treadmill is better than an exercise bike or stair climber. It's the closest thing to actually going outside and running.