Once the VPN is set up, using it is simple: When you want to connect to your VPN, you re-open Settings, tap VPN, select the configuration you want to use (if you have more than one), and tap the VPN on/off switch. Enter a password (if it isn't already saved in the configuration). A couple of seconds later, you should be connected.
At that point, you can connect to your network and do whatever you need to do, and it should just work. The only visible sign that you're using a VPN at all is the small icon in the upper left corner of the screen. When you're done, go back into VPN settings, tap the VPN on/off switch, and the VPN connection will be terminated.
If you're using an SSL VPN, you can take advantage of "VPN on demand." Then, you don't even have to turn it on. Whenever you try to access a site or a resource behind the VPN, the VPN will automatically start for you and quit when you're done.
As with so many other things, Apple has done a good job of making some ferocious technology simple to use. Setting up a VPN on the back end may be a truly tedious and annoying process. Connecting to one from an iPad is anything but.