Can't say enough good about it. So much time, energy, and money saved. I would occasionally do a full day's work over GoToMeeting, or, rather, a full night's work when crisis time inevitably hit. Beats hauling ass to the client's office at 2 AM. WebEx consistently let me down.

Does it really scale to 100 participants?

Gotomeeting is for smaller, interactive meetings; Gotowebinar claims it handles up to 1000 participants.

Yes, and beyond.

I just did a joint webinar with 100 attendees over my Verizon USB dongle on GTM. Worked great.

We had an extremely poor experience with goto webinar (I wasn't directly involved, I just heard about it second hand). Apparently the presentation was seriously disrupted by technical issues.

I work for Citrix (the company behind GTM) though in another team, and we use it internally and it works beautifully for very large All Hands Meetings too.

Not shilling my company, but I really like GTM.