I'm with ColinsMum on the Snapcircuits. We got them for DS7 when he was around 4, I think, and he played with them for a while when he first got them, but has shown little interest whenever we pull them out again. We got him the Lego NXT robot when he was 5, I think, and that has gotten a lot more play time. We have only child syndrome...otherwise we'd stop buying these gosh-darn expensive toys!
He still plays with his regular old Legos though. They are a classic for a reason.
ETA - we have only one Lego set piece that has stayed in its intended shape (the VW bug - he must like it because his fabulous mom drives one). Everything else has been morphed into so many things I cannot name them all. At first we were disappointed that DS would destroy the Lego creation right after building it, but now we appreciate his creativity. The rule now is you must build what comes in the Lego set at least once, then the blocks are fair game.
Edited again to add: I really shouldn't read these threads backwards, lol! I just read lucouno's description of the instructobot, and as I was just skimming, I started thinking, gee, that must come with a more recent version.
But I agree that the current instructions worked quite well, and the programming is very simple. (Yes, a 5yo can do it.) Our dog also really enjoys finding and then hiding the little balls that the robot throws...