The guys associated with the prism project released a book called 'Developer's Guide to Microsoft Prism'. It was released as a free PDF so don't pay for it unless you want the hardcopy. (Look in the table at the 'Downloads' section on http://msdn.microsoft.com/en-us/library/gg406140.aspx[^]) I suggest you read it even if you don't end up doing anything with Prism.
Prism is the ultimate MVVM toolbox. It has everything which makes it much heavier than something like MVVM Light. Which is why you should read the book and understand the advantages and disadvantages of the framework. In short, where something like MVVM Light is designed to help you build an application that follows the MVVM pattern by providing resources that make the task easier, Prism is designed for building composite applications using the MVVM pattern. That is an important difference.
Like many other frameworks and patterns, you can pick and choose what you want to include so while the whole shooting match is pretty big, you don't have to use all of it. I like it, especially for large projects where the ability to break the resulting app into modules that are located and loaded at runtime provides a big advantage. But I also use MVVM Light a lot as well when I don't need the horsepower Prism provides and I just want the MVVM pattern.
Regardless of whether you ultimately end up using it, you should probably learn about it. At least then you would be able to recognize on your own when it could/should be used rather than relying on the infamous 'I have heard...' decision making process.