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.