I run openSUSE on a work laptop. 

- The office has an HTTP proxy server, so whenever the laptop is in the office,
I plug in an Ethernet cable, run YAST, set system proxy settings to the office
proxy server address, log out, log in and start working.

- At home, I use wifi and run a proxy server in my router. When I use the
laptop at home, NetworkManager automatically finds the correct SSID and joins
it, but I still have to run YAST, set the proxy settings to the right address,
log out and log in again.

- When I'm travelling on business, the laptop of course encounters many wifi
networks. If I want to use wifi in a hotel or at a conference centre,
NetworkManager joins the network without problem. But I still have to run YAST,
clear proxy settings, log out and log in again.

I know this is because http_proxy is a system environment variable, and the way
X is run means it inherits the environment variables of the underlying shell.
But it would be really nice if the system was able to choose a proxy server
address based on which network interface or SSID is active, or even just have a
list of "proxy profiles" from which I can choose the most appropriate one
without having to log out and log in.