You betcha it is useful! I used this over the course of the last year whenever an idea sprung to my mind. It's imple and easy, and when I just want to see what happens when I "hook that to that", I don't care which transistor it is!
This is a perfect little applet, including loads of sample circuits. Also a helpful appliance if you want to show the schematic of a circuit, but have neither paper+scanner or software at hand.
Also check out his 3d interference applets and 3d vector visualisation - extremely neat!