I think Randy's quote from the WSJ story sums it up:

"The data can be misleading," said Mr. Petersen, founder of InsideFlyer.com. He'd like to see more data, including numbers on how many customers made requests but couldn't find seats.


Of course, what would be really meaningful would be a promise of at least X seats per flight in each class. But airlines will be loathe to do that absent congressional action.

As for personal observations, I would for the most part agree with GUWonder. I have had little trouble booking free trips on AA or UA; however the rest including US lag far behind.

A couple of years ago I tried to book a free coach trip on US from SFO to PIT to do a midwest baseball stadium tour. Now PIT is not exactly Maui or Europe, and my dates were flexible. Lo and behold there were zero flights available (incl. redeye). Not a single seat on a single day in July or August. Basically US customers in SFO were unable to to book a free trip during those months to a US hub (even tried going through CLT or PHL). So I would have to rank US near the bottom, though on occasion you can find *A awards via UA.