Echelon: What is the straight dope?

Is there any hard data on the capabilities of Echelon? Most of the stuff on the Internet are wild speculations that come from tinfoil hat communities or other dubious sources.

Also, has Echelon found any useful information (like terrorist communications) so far?

Echelonwatch seems to me to avoid donning the tinfoil, the substantive portion dealing with reporting the findings of other governments’ publications on ECHELON. They also have some reports on what echelon has been used and misused for.