If the party requiring support is (unfortunate enough to be) using Windows, then a VNC solution might be the best balance between function and security. VNC over and SSH tunnel would be best, but holy mackerel would we ever be making this more complicated than it needs to be!
I have found that some of these web-based solutions, using uPnP to avoid the necessity of proper port forwarding and/ or proper firewall configuration, and are "on all the time" present a bit more of a security issue than I feel comfy with. If I am going to administer a system for someone, I need only configure it's remote desktop for tools that I will be using.
The Windows systems I (am unfortunate enough to) administer all run the TightVNC server, and I just use the Screen Sharing app on my Mac to control their screens.
For Linux, *NIX and Mac systems I just log in using SSH to perform updates and edit config files, or use their system's built-in VNC server if they need to be walked through something.