How stable is GNU Smalltalk? How scalable? How fast?
It is quite stable. There are bugs for sure in the virtual machine, but unless you're unlucky you shouldn't be affected. This is especially true for the stable branch, where only changes to improve stability are made; development releases are by their very nature less stable.
It is quite scalable, even though the garbage collector performs worse once you have more than 150-200 MB of live data. Very long strings also do not perform as well as they could. In general, object-orientation makes it easier to pick good data structures (such as streams).
It is quite fast. While GNU Smalltalk has an experimental just-in-time compiler, even the bytecode interpreter should be faster than most other scripting languages. However, GNU Smalltalk's base classes are written entirely in Smalltalk (unlike Python or Lua, for example). While this gives more flexibility to the programmer, programs that heavily use dictionaries may run slower than the equivalent programs in other languages.