Smalltalk is an object-oriented, dynamically typed, reflective programming language. It was designed and created in part for educational use.
Pro Elegant syntax fits on a postcard
The syntax was designed to be easy enough for children to learn. Beginners can learn the language rules very quickly and then focus on programming without fighting the syntax at the same time. Things that have to be baked into the grammar in other languages are simple message sends with block arguments in Smalltalk. Expressions have only three precedence levels to worry about.
Pro Superb Integrated Development Environment (IDE)
All tools (Inspector, Browser, Debugger etc.) are written in Smalltalk and are live objects in the environment. All sources are present, so that the tools can easily be studied, changed and experimented with.
The same goes for the other components like the compiler, OS-Integration etc.
Pro Powerful integrated debugger
You can edit code and swap it in while the program is still running after an exception has already been signaled, or restart from anywhere in the call stack. You can inspect and modify the state of any object. Some Smalltalkers write unit tests and then program exclusively in the debugger.
Pro Inspector makes objects transparent
Programmers must make detailed mental models of the system they are developing. Bugs usually happen when the mental model does not match the actual system. This is one of the greatest difficulties beginners have because most systems are so opaque. It takes a lot of effort to see what's really going on. But in Smalltalk this is much easier, thanks to the powerful tools included in the environment, like the object inspector.
Pro Incremental compilation
Smalltalk provides an extremely fast code-compile-run-debug cycle. You don't have to stop and reset the world to tweak your program, since you can compile one method at a time while the environment is still running. This is great for beginners to experiment and prototype ideas.
Pro Save and restore virtual machine image
A Smalltalk environment can save the state of a running program and later restore and resume execution. This includes the internal state of live objects, multiple thread stacks, and debugging sessions, making it easier for beginners to take the exact problem to an expert for assistance.
Pro First-class functions with lexical closures
Also known as "blocks". These objects contain reusable snippets of code and as first-class objects they can be passed as arguments to other methods or blocks and also returned from them. "lexical closures" mean they retain access to the variables in the lexical environment they were written in, that is, in the surrounding code.
Pro It invented a lot of stuff
Smalltalk is the inventor of Just-in-Time compilation and the MVC concept, refactoring through their so-called refactoring browser and it was also one of the first adopters of a language virtual machine, closures, live programming, test driven development, an IDE and the development of GUI`s.
Pro As a first language, almost forces you to learn OO design
Hybrid languages (e.g., Java, C#, C++) make it easy to slip into procedural thinking. Smalltalk's pure OO approach makes it hard not to think in object-oriented terms. In addition, since the entire IDE and runtime components are there in the image for you to browse, you have plenty of examples of good OO design to learn from.
Pro Provides a functional way to interact with objects
Many languages today use object orientation, while the most of them stock on the half way in that perspective.
Smalltalk sees literally everything as an object and this includes things like the classes and primitive data types. There is are zero control structures such as selection and iteration, since all is done by sending messages to objects.
It use a lot of concepts from Lisp in order to provide a nice experience for this pure kind of object orientation.
It provides immutable data structures, closures, anonymous functions and higher order functions, while all those functions are objects. This is what makes Smalltalk so simple, elegant, and easy.
All this counts for Pharo, while other implementations as Amber are probably feature complete to it.
Con OO is becoming obsolete
Smalltalk did it best, but the whole paradigm is a poor fit for the expected future multicore processors. Isolated mutable variables with no compile checks is a recipe for race conditions in multithreaded code. Beginners would be better off learning a functional language.
Con Not common
Smalltalk missed an opportunity to become mainstream when its implementations cost $5000 per seat versus $0 open source. New open source implementations (Pharo, Squeak) have minor corporate backers but not yet an IT behemoth. Direct jobs are scarce (but indirectly Smalltalk experience is very well regarded). Online communities are relatively small.
Con Non-standard arithmetic ordering
Since every operation is considered a message sent sent is a specific order, all arithmetic operators have the same precedence. E.g. 2 + 3 x 4 translates to 2 + 3, and the result is multiplied by 4, giving an answer of 24 (instead of the correct answer - 14). Once you are learn this, it can easily handled using brackets, e.g. 2 + (3 x 4), but still a momentary suprise for beginners.
Flagged Pros + Cons
Out of Date Pros + Cons
Pro Interactive Test Driven Development
After the Smalltalk community invented TDD , many languages copied, but they missed the interactive nature of the original. Working with live objects as write the tests  enhances the feedback loop to flesh out an API.