Actually I used the PEAR::SOAP extension, mainly because it should ease the transition when I need to port it to the native PHP5 extension(which seems to be looming over my head anyday now)
The biggest issues I ran into were interop related. Namely interop from a PHP server, to a .NET client. All the PHP libraries I've looked at thus far, have poor (at best) support for WSDL generation of Doc/Lit based services, which is about the only way to interop with .NET.
The lack of documentation thing seems to be a common issue for all the PHP based SOAP frameworks, which makes it a tough thing to learn from the ground up. But such is life 🙂