Is it worth upgrading yes and no.
Yes there are tonne of things that make life easier with PHP 5. Better OOP is just one and SimpleXML is another.
However there arent many hosts to utilize these features.
Bugs well so far Im yet to see any however I havent used things like SOAP or other not as used functions.
PHP 6, defiantely not PHP 5 is basically only just out 5.0.4 is the latest unless thats changed in the last few days its quite stable I feel and should be getting ready to be used on Hosts relatively soon Id imagine.
Also in PHP 5 there are some new Sting functions [man]str_ireplace[/man] is the first that comes to mind.