Wow! I can't believe that my experience with MCI is so different than the previous posters. I've found their rates extremely competitive (for US and International calls) and their customer service friendly and knowledgeable.
Once I got burnt by using my calling card outside the US to a number also outside the US (I knew it was going to be expensive, but I was surprise that it was going to be *that* expensive). I called Customer Service and I got a 33% off the bill as a gesture of goodwill.
BTW, I have a $3.50/mo plan that allows me to make 7c/5c calls to Canada on weekdays/weekend in addition to AA miles.
I usually change companies every 6mo or so to keep getting their bonus (current one I have gives me 1000mi per month for the first 5mo in addition to 5mi per dollar spent).