IMHO it's well worth the time. Smalltalk obviously has a very pure approach to OOP (though I'd shy away from saying that it is the only genuine OOP-language, or even the only reasonable approach to OOP), and I think that learning Smalltalk helped me really get a handle on OOP.

It's not necessarily impractical, either. I know that if in the future I had occasion to develop certain types of Windows-specific applications (mostly notably, but not restricted to, the types that VB is aimed at) I would use Dolphin Smalltalk in a heartbeat (and I have used it for a few such in the past). More cross-platform enviroments like Squeak and VisualWorks are also fine Smalltalks.

The book I recommend for an introduction to Smalltalk is Chamond Liu's Smalltalk, Objects, and Design.