If they'd included 100-200 puzzles I would have played the game until I finished them all. Of course, I am a Mechanical Engineer, so this stuff was right up my alley. Since there were only 25 puzzles, I assume that this is the main focus of the game. The books included in the package include instructions on how to make a variety of different widgets. They also tell why they work.
Parts used in the widgets include timing devices, switches, and logic gates.
You can also upload your widgets to Maxis and download ones that others have created. I also tried to import some stand-alone widgets I downloaded from the Maxis web page but didn't have any luck. Every time I tried I just crashed the game. I'm not sure why someone would upload a stand-alone widget without including the .WGT file, but most of the files that were uploaded seem to be of that format. I didn't realize this until shortly before press time, but this gives me a little chance to rant.