It works mostly like traditional FP but through the smartphone app.
Yes it works for both parks.
Most people find it worth it especially if they want all the photos (which are included with MP).
I think MP is a better deal for smaller groups - like 2 people. The cost is cheaper and can be the same cost as if you want to buy Photopass anyways. As the group gets bigger, then its value may not be as much at $10 per person.
Read this thread for more information: FastPass, MaxPass. Watch the two videos -- sometimes it's easier to see something in action. You can also do a search in this forum (search bar in the upper right under your name) for more threads with lots of information and tips on MaxPass.
Yes, you can buy it for one day to see how it works for you. Whether it will be "worth it" for you depends entirely on you and your touring style. Many people use it and love it. Many other people stick to the paper FP system and love that. Both work well if you understand the basic FP system (this is key) and know how to use it to your advantage. If you decide to get MP, make sure your phone can handle the latest version of the Disneyland app, that you have a good data plan or that you are willing to rely on the (not so great) free park wifi, and that you have battery back up.