Randy's article is very good. A few other points though that I think should be emphasized:

1. Inflation. Awards are becoming more expensive, miles-wise. Your miles don't earn interest sitting there unused. You money will.

2. Availability- I have found that USING miles is VERY difficult in the past 6 months. I have had a heck of a time booking any ticket anywhere where I thought it might make it worth my while to use miles. CO has made it impossible to go to Europe without using EasyPass. Thus it makes those miles worth a lot less than before.

3. Program differences- the one big exception here is Starwood (although we are talking about points, not miles). A lot of people convert their Starpoints to miles, but I generally wouldn't.

Why? No blackout dates. No capacity controls. You can use your point whenever and wherever you want. This makes these points MUCH more valuable than your typical mile.

4. They make up the rules. And they can change them when they see fit, with little notice. Don't forget, the airline is in charge. You are not.