It's very much like an early version of Smalltalk: monochrome, with an early-80s style GUI. There's a popup main menu with a surprising number of options, plus several editors, debugging tools, and a Smalltalk-like 'Transcript' window (for output). The system is image-based, so the running image could be stopped and restarted, even from a different machine.

More intriguingly - and again, analogous to Smalltalk - Interlisp is more than just another Lisp dialect: it's a complete operating system running atop a Lisp VM, with Lisp as the system language, from top to bottom. Awesome!

By today's standards, it's clunky (as one would expect), but if development had continued, I suspect the comparison to Symbolics Genera would be the analog of comparing a GUI desktop environment to a console-based one. Considering the richness of modern environments (have you seen Pharo or Squeak?), that could've been a seriously cool Lisp development platform...

Incidentally, ParEdit is based on Interlisp's SEdit (in Taylor Campbell's words, 'a real structure editor, not a cheesy imitation like paredit').