I've been using Zend Studio Neon for the past three weeks and I have to say, for something that is free (beta software) it is quite good. Compared to using VS 2005, Zend Studio Neon has its advantages, since Visual Studio doesn't handle PHP well. Zend Studio Neon also has areas that I didn't like.
His likes included the fact that it's not Eclipse (technically) and that it supports PHPUnit and PHPDocumentor out of the box. Dislikes included the fact that it might not be free in the future and that it lacks FTP support.