I think it's a good language. It has simple syntax (only 13 syntax rules), great documentation for the API, the language, core commands, and all the packages shipped with the core release. It's very flexible so it can be used in a lot of situations: Embedded into a program as a domain-specific language (DSL), scripting engine in its own right with very complex software written in many different styles, evaluating untrusted code (with Tcl Safe Interpreters).

Really the only reason to pick Python over Tcl is that more people have picked Python over Tcl in the past.