I've been silver for two years now. Its been totally worth it in my opinion. I don't travel all that frequently, and I have a mix of domestic and international leisure travel. I've been upgraded on NW metal 75% of the time, and on even on Continental (out of EWR) 50%, so I'm very happy with it. But the abilitiy to pre-board when I don't have an upgrade makes life more comfortable as well!

I'd say its definitely worth it. I'm aiming for Gold this year.

Bad bet on Korean Air suggestion, IMHO
 

Regarding this:

"Do you think it would be feasible to take the KAL non-stop and just not enter a FF# and then hold on to the BP / ticket until NW is part of Skyteam and then ask for credit for the flight in WP? Or would this get negged as a flight having taken place prior to the partnership commencement?"

No, I would not do this.

oddly enough, i've been upgraded on CO with my WP Silver more than I have been upgraded on NW. I think I'm 10/12 with NW & 6/7 on CO (higher percentage on CO). I even scored FC on CO IAH>LAX and back! Never thought that would happen (of course, back was redeye though).

anyway, it's worth it. however, my status expired and i'm back in the boat trying to get back up to silver myself. i'll be doing some mileage runs, if need be, at the end of the year.