I'm still trying to figure out the circumstances you would use it but I've just been playing with a web based SIP softphone. I'm amazed how easy it is to use. You just browse to the webpage, say "yes" to an activex install (your browser does need activex) and then type in your PBX ip address, extension and password and you are going. This softphone will even remember your settings. It also allows you to determine which speaker and mic you will use.
I didn't do extensive testing with 3CX but it seemed to work decent at a basic level. I'll let more indepth testing for you. I do understand 3CX is working on their own webbased softphone. They have already released an alpha (don't quote me) of it as a simple windows softphone.
So I guess even a windows based terminal could be a softphone for the IP PBX of your choice.
If you want to see my first impression video about the web based sip softphone, goto http://landiscomputer.com/3CX.