I am studying CCNA and MCSA and PT has proven to be more than useful. It allows the hands-on experience as well as learning the theory to go behind it. For instance, you may know how to configure RIP or configure a router port to work as a VPN but you can only secure your knowledge by actually doing it. Although my main experience is with PT 3 I have recently been given PT 4.
The only problem with PT3 is that you can not configure devices very extensively, for instance you can assign IP addresses to ethernet ports, serial ports etc etc, but you are not able to configure RIP or any other protocols which you need for CCNA Semester 3 & 4. Also you are not able to configure switches very well either, which isn't very useful later on in my course.
Havent really been on PT 4 a lot. When I do get chance I will let you know the difference.