I used to use GoToMeeting about a year ago, but that was too expensive at $49/mo.
Some things I liked about it (in the context of learning computing) include:
- I could give point/click access to a user ...and quickly take it a way.
- My mouse would override their mouse.
- I could record a video of the session which was handy for complex configurations of software, etc. for later review (tutors charge by the hour).
I have very briefly searched Google to find a few competitors; one is free. You could discuss any competing service. The question is how do they compare? And, what was your person experience like?