Although I live in UK now, most of my NW flying is US domestic, and most of it between non-hub cities where NW has a presence but is not the primary airline. Cities like MCI, ATL, IND, AUS to name a few.

When I lived in the US (until 2002) flying patterns were similar except with one or two trips to Europe and a the odd trip to the west coast.

If you look at the marginal benefits, for a flyer like me the jump from base member to Silver on NW is probably the biggest jump you make in terms of benefits

Even though I'm going for gold and maybe platinum, I can tell you that after about 5 yrs as a NW Silver, it is probably the best "value" elite status out there.

I've found the best benefits to be, in order of usefulness to me:
* Upgrades - I'm 22/23 since 1 Jan 03 on eligible flights
* Priority checkin - I always use the first or business class checkin queue whereever I go on NW or its partners and this has probably saved two days of my life when you add up all the time I could have been waiting! For example, at Gatwick, the queue for checking in economy typically has 100 people in it when I get to the airport, and WBC has about 3 people in it. When I used to fly out of MCI a lot it also made a huge difference. Now with more kiosks I'm not sure how much time you save. Last time I ckd in at LAX they were having everybody use the same kiosk line, 1st class or not. A rant for another time...
* Priority Seating - It's so nice to book a trip about a week out, pull up the seat map and find the plane 2/3 full. Meaning the back 2/3 of economy is packed solid, but plenty of open exit rows and seats in the first rows of economy. As a silver these are yours for the asking.
* Priority Reservations Line - Although SEs don't get their own dedicated agents like GE and PE, this relatively new feature is great. When you ring the SE number it puts you at the front of the call centre queue. I've never waited more than a few seconds since calling this number.
* Mileage Bonus - Last but not least, you do get a 50% mileage bonus, which certainly doesn't hurt any.
* International op/ups - Just kidding on this one. If you even think about enquiring about your chances of getting one at MSP or DTW, the GA will make you feel about 2 inches tall and might even take away your birthday. At best you'll be snapped at. In this case your silver elite status is a boat anchor, you may as well be a base member on Uzbekistan Airways.

As I said before, I think the jump from base member to Silver in WorldPerks is probably the biggest increase in marginal benefits that you can get with 25k miles on any airline flying. I have been a very satisfied silver elite for five years. But the caveat is that this applies to my flying patterns and might not apply to yours.

As stated by others, if you will travel frequently between hubs (high elite load), or exclusively internationally (WP Silver has little or no int'l benefits), or between a lot of small cities served only by CRJs and Saabs, then Silver on NW might not be worth it for you.