As features go, this didn't prove to be super successful (it's no longer shipping with Firefox).

I do still think it's a valuable interaction style for being able to activate less-frequently-used features all from the keyboard. It's possible that the fact that web browsers also have the JS Console is just too much, because the JS Console has a command line that people are familiar with. The interaction model is very different, though. GCLI made it quick to type commands, whereas the JS console requires valid JS.