All encryption can be compromised with enough computing power, having strong passwords is the key to stop attacks. A 128 bit AES key would take billions of years to brute force.
WEP and some WPA can be exploited much easier than a WPA2 connection, WEP was flawed in many ways and WPA was a backwards compatible plaster for WEP.
All WiFi should be WPA2 which uses AES 256 bit encryption for maximum security.
Any site that would share guest access with the rest of the network is asking for trouble.