I'm not sure it would allow logon/logoff because the user-end has an app to allow them to end the session or end your control over their computer (and allow you to just view), but it allows you to use UAC prompts and enter passwords etc.
Yea, with Windows XP at least, a user needs to be at the computer to initiate a Remote assistance session. I've NEVER gotten Remote Assistance to work properly.
One of the annoyances I've run into with TeamViewer is that it blanks out the screen whenever a UAC prompt comes up, so if you're using TeamViewer and try to install something, its impossible to do it remotely. They have an option to enter admin credentials when connecting which should help to over come this problem, but its never worked for me. I've tried to use TeamViewer to remotely install LogMeIn, but it just wont work since LogMeIn requires a UAC prompt (if not logged in as admin).
LogMeIn is a much better option. You don't need to open ports on the firewall, or have to remember the IP address. You just login to the LogMeIn website and if the computer is online, you can connect (LogMeIn takes care of IP stuff).
Its slow, but it works. LogMeIn free works great. I have it installed on about 80 computers and use it daily.
LogMeIn also has Hamachi, which is easy VPN software. Install it on two or more computers and it assigns them 25.x.x.x IP addresses on a private network and you can share files between them. You now have to be logged in for Hamachi to work. It used to run as a service until they go a little greedy back in November.
I actually use Hamachi to tunnel Remote Desktop on occasion. Remote Desktop (Requires Win Pro+) works better than LogMeIn, but needs a hole in the firewall (very insecure) and you have to know the IP address.