This is a fantastic discussion, slightly old but not seriously so:

http://runasradio.com/default.aspx?showNum=81 (30 min or so podcast)

Many of the metrics can be trusted and used, not in terms of hard thresholds, but in terms of what's normal. For example's Paul's example of PLE, say my production server usually has during business hours a PLE between 10000 and 15000, I check perfmon today and find that it's sitting at 4000, that is a cause for concern. Not because it's below some magic number, but because it's far from what's normal for my system.

p.s. The PLE threshold at 300 was relatively sensible 10 or so years ago when servers had small amounts of memory. These days, not so much. A PLE of 300 means your entire buffer pool is getting discarded and fetched from disk in 5 minutes. On servers that can easily have > 64GB of memory, that's not great.