One thing I have noticed when flying NWA metal on another carrier's ticket is that the fare bucket I booked gets converted to a much lower one on NW.

For example, on my SEA-SYR $118 Delta B fare mileage runs a couple years ago, the Delta B fare (Just under Y, 2nd Highest economy, 150% EQM bucket) got converted into a NW M fare which is just a mid range economy.

Similarly, a Continental V fare code share (mid range economy ticket) got converted to a Northwest T (the 2nd lowest economy bucket).

Since Elite upgrades go out to the highest fare buckets first, it seems people on another carrier's ticket flying NW metal will have a big disadvantage.

On a lighter note, I have been EUA'd more than 5 days in advance flying NW.... even when just a Continental Gold, when flying through smaller markets such as SYR, BHM, LIT on a cheap fare. (Then again, I've had seats like 37B and 35E when flying as a Gold or Platinum between SEA and MSP/DTW on high (i.e. Y or B) fares.