WN's Rapid Rewards Best:
-Award availability
-Double credit on website
WN's Rapid Rewards Worst:
-Companion pass too restrictive
-No international partner on which to use awards
-Not a great deal of perks (pre-boarding for CP holders, or lifting the 1-hr check-in requirement for CP holders would be nice and would not cost a thing).
UA Mileage Plus' Best:
Star Alliance
-1K SWU certificates
UA Mileage Plus' Worst:
-No double upgrade mechanism
-Not enough SWU for 1Ks ( 6 a year)
-Lowest International fares not upgradeable with miles
Overall, I think the threat of unilateral devaluation of miles is a large drawback of all programs. We can all wake up tomorrow and find out that our miles are worth half of what they are today.