I always felt that the syntax was painful to work with - it reminded me of scripting with csh way too much. That's not to say Tcl/Tk didn't have areas where it would shine brightly back in the day. Those would be Expect and Tk. When you needed to 'automate' accessing hosts or devices via telnet, Expect could do that and then some. And Tk was probably the easiest way to create a GUI on the fly for unix and linux. I know that most of the management and configuration apps for UnixWare 2 and 7 were written in visual tcl, a curses enhanced version of tcl, which made dealing with UnixWare less painful.

Unfortunately, there aren't really any big killer apps written with tcl, and I haven't seen any major new books (a 2nd edition of Ousterhout's book and one or two others). I suppose it's better than the situation that Eiffel is in, though.