The question is one I have heard a lot. And it's one I've spent a lot of time trying to satisfy!
As I see it, there are several levels of using PowerShell, and therefore several levels of learning to be done.
First, using PowerShell from the command line, e.g. to set a permission in Exchange, or enable a user in Lync Server 2010. The concepts of cmdlets, pipelines, and objects start you on the road. You need to learn a bit about the syntax of PowerShell, and the simplest of scripts and a lot about discovery.
This leads into writing scripts, particularly production oriented scripts. You need to know the use of functions and modules along with PowerShell's scripting based operators. And you need to learn a bit about debugging, formatting, and how to dig into WMI/COM and .NET.
At that point, you have a good template for how PowerShell works. You then need to apply that knowledge into the cmdlet set of one or more applications. For example, there are over 400 cmdlets with both Lync 2010 and SharePoint 2010. The Active Directory cmdlet set for Server 2008 R2 is over 70. Etc, etc.
To make that transition, you need to apply your discovery skills to look at the nouns that each cmdlet set provides. Those nouns represent the objects that the cmdlet set operates on - and by now you know all the verbs (Get, Set, New, etc). Now beyond that, and you stop needing to know PowerShell and you start needing to know the application. PowerShell can help to focus you on the objects and provide easy ways to learn more.
To help this last bit of learning, there are a a number of blogs and web sites. Google is your friend. But to me the best site so far is the Lync PowerShell blog at http://blogs.technet.com/b/csps/. There is lots of very good information and it's both well presented and very well written. As it happens, the current team is the brains behind the Original Scripting Guys - and it shows (in a very nice way).
So some thoughts on how to think about learning PowerShell. Please feel free to follow up with questions or thoughts of your own.
Thomas
I study PS by reading Tobias Weltner's book http://powershell.com/cs/blogs/ebook/default.aspx after having watched all of the Microsoft Webcasts (I take this opportunity to thank my Father and Mother (both passed) for sending me numerous times in English speaking countries when I was at school :-)
Tobia's book is one of the best e-books out there. I am grateful for his work as I've learned a lot from them.
I also recommend 'real' books, including those by Bruce Payette and Lee Holmes.