I picked up a Trek 4500 for my first real MTB that wasn't $80.00. Ridden 69 miles on it this week - mostly on trails. It was $419 at my local shop. No complaints.
Also got a Trek 3900 for the other half - she just got it today and put 7 trail miles on it - also a great bike.
If I was going to spend $500 - $600, I'd likely get the Trek 4900
Bryan