Elluminate

Quick and easy introduction: A recorded Training session:
http://www.elluminate.com/Training/Documentation/Detail/66/?id=227 

Lesson learned: Describing a click takes many words and can make things appear more complicated than they are. Showing it on screen is much easier to understand.

Strength:

• Explaining the use of software in an interactive synchronous way.
• More commitment in a meeting compared to telephone conferences, as everyone sees the list of the participants.
• Working with groups in diverse places.

Weaknesses:

• Even though the features seem easy, it takes some time to learn to use them without irritating yourself or the students with the technical features.
• Technical aspects of access (line capacity, costs, company policy,...)
• It is likely that not all participants always have the same experience and capacity to use the tool. Sorting out problems will take some part in many sessions.

These types of software tools need some training and experience to be comfortable with. Otherwise sorting out the technique might overlay the purpose of the session.
I would choose it for groups that will stay together for a time. E.G. training that goes over some weeks or teams working in projects, departments...
I would not use it with occasional participants, where I have no information about their web conferencing literacy. 
I suspect from the feedback of some friends using web conferencing, that most web bases conference tools are primarily used to do presentations online. The interaction part is reduced to vocal feedback with some occasional chat text (advanced users).