We've tried a few different ones in our company, with WebEx always coming out on top when it comes to functionality and reliability. Unfortunately I think WebEx is the most expensive alternative.
One advantage with WebEx is the very simple network setup, it doesn't require lots of firewall rules etc to work. Most of our clients are large banks with a strict deny all Internet access policy and only allow access via filtered proxy servers, webex still works.
We had a very poor experience with GoToMeeting, half our clients couldn't get it to work. When raised with GoToMeeting they sent us a very complex firewall setup doc which obviously no banks are interested in looking at.