NW often offers Perksaver First when Perkpass Economy is sold out for domestic routes. Sometimes people don't think of checking for Perksaver F before just booking Perkpass Y which is a real shame! Why spend 5K more miles to fly economy when you could have saved the 5K and flown coach?!

CO's website for award travel is nice - it lists economy/first, perksaver/perkpass (equivalents) all side-by-side so you can see how many miles it takes for each option.