I'm new to all this, and am looking for some friendly advice from mileage uers.
I'm thinking of replacing my primary credit card with one that offers free miles.

My wife and I don't travel a ton, but we would like to start going on a couple of domestic and international trips a year.

Is my thinking correct in that getting a credit card that earns miles, we will be able to do this??? Is it as legit as it sounds?

United offers 20,000 miles just to sign up. The only down side to this that I can see is the high rate (14.94%) and annual fee ($60). Am I missing something???

Also, could any of you tell me what the true downside is to using free miles earned on your credit card to travel? Is it more difficult to get tickets?

For example:

My wife and I want to revist Scotland, but we don't want to pay the $1700 for airfare that we did the first time around. Denver to Scotland is about 5,000 miles. So my wife and I would use 10,000 of our miles to get their and pay nothing? or just taxes? Is this a viable way of going about this???

Please help. I'm new to this idea and it all just sounds too good to be true.