While I think these pass plans were attractive at one time, they are way overpriced at today's rates.
If you currently fly 50,000 miles per year, and wanted to try this payment scheme out for the next 2 years, it would cost you:
$25,500 (Base 2-Year, 25K Pass)
+$12,500 (Excess 25K miles for Year 1)
+$12,500 (Excess 25K miles for Year 2)
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$50,500 (Total investment for the entire 2-year period)
However, you are given a free Admiral's Club membership, and most importantly can travel at a moment's notcie, as your pass is considered full fare payment.
Where it gets a little confusing is when you want to fly international or in another cabin. You draw down your miles faster by travelling in First or Business, but oddly enough, you get international coach for fewer miles charged against you.
So, if you regularly fly last minute flights and pay full fare, then this might be worthwhile, but with the availability of discount fares, I can't imagine buying this plan.