+1 for Teamviewer

Also agree- 98% working and much easier than anything else out there. 
Was using a VNC callback app but decided teamviewer is much easier in the long run. 

Its expensive but it is worth it. And no problems with UAC

Only issues i find
 - If you buy a version, your clients need to run that version, not the latest version
 - If another software vendor has installed teamviewer, the client needs to exit the tray app before you can run yours

Other than that it works brilliantly.