I have been using Bomgar (www.bomgar.com) and can not be any more pleased with the solution. I think it is great. You have total control over it. I can support windows, Macs, and LInux desktops as well as iOS, Android, Blackberry and windows mobile devices (great with customers that have handheld barcode scanners).
I have 5 reps and I can share my session with any and all of them plus transfer it to them if I no longer need to be on the session. For those vendors that don't have access to a client I can invite outside vendors to connect using the software as well and for just that session they have the same control I have. I also like having the ability to have predefined scripts. You can run a command shell without logging into the machine. It is handy if you do not have a login account or if your account is locked to be able to run a command shell/script to unlock or create a user to log into the computer/domain. It also allows for recording of the session for playback. Great when you try to figure out what something looked like before you did something on the customer machine. It can also do surveys afterwards to get customer feedback.
I think you can even set it up to have multiple domains so if you wanted to you could buy one enterprise version and split it between multiple resellers. I have most of our customers pinned so I can get into their system anytime I need. I am paying a fraction of what I use to pay with the GotoAssist.