For me it's not mileage, but age. If serviced correctly the engine/transmission will outlast the rest of the vehicle. When they hit the 12 year mark I start looking for a new vehicle. The interiors start coming apart, AC fails, lots of little things start stacking up. I just rolled 180k and haven't had to fix anything I didn't break through stupid. The duratec 3.0 is known to run almost forever.
My roommates 2001 Malibu however, it's got some internal engine issues, pretty sure the LIM gasket needs replacement, the cam position sensor is bad, the EGR valve went bad. Alternator died. Suspension was totally and completely shot at 80k. The shocks had no damping at all and it was scary over 40mph. The older they get the more iffy they become.