I am disappointed by PDT for Eclipse. The current version strikes my rather old machine with "just" 512mb to death. Building projects is not possible even when assigning eclipse all my available memory, because of an Java Heap Error (See this blog post). This effectively hinders you to use all the completion and hint features, because PDT does not know about any functions, classes and their phpdoc descriptions.
PDT is also missing some very important features. Marking all occurrences of a function, class, constant or variable on click for example. PHP Errors and Warnings are display in a way so that you won't find them and if you do, you won't understand what they mean.
So I am going back to my easyeclipse with php combination, which always worked well, the only problem being that it is no longer under active development. At least it has all the features I need for now.