I believe ZombiU worked wonderfully to show off the Gamepad. Nintendoland gave some basic ideas at how it can be used and Rayman makes good use of it with Murphy. Tekken let's you cheat (RAGE) and Batman makes the game ultimately more smooth when using the TV with the gamepad. Monster Hunter allows you to arrange your HUD differently which can make things simpler or prettier (a hudless screen) and makes it easier to communicate when online given it's built in Microphone and Touchscreen Keyboard. Most Wanted also allows it to add to the gameplay, however, I wish the Gamepad could be used for more than just God Mode.
Does the gamepad add to immersion like Motion controls did for the Wii, or does it just feel like a tacked on gimmick?
I wouldn't say a gimmick. But I also wouldn't say it is as revolutionary as the Wii-mote.
The Gamepad serves a great purpose for those that share a television with a significant other, with kids, etc. Most people in this situation swear how by how wonderful this option is to keep everyone happy. For many married individuals or parents, this is a massive advantage over the competition.
I am not in the above scenario. However, I feel by looking at my Wii U retail and digital collection, I have about 20% Nintendo titles. I feel that the 3rd party developers have used it in ways that are beneficial, such as mapping weapons to the touch screen and having maps there to not eat up TV space.
As far as people saying the controller itself is poor, I highly disagree. I have played using the game pad, on certain weekends, for more than 12 hours. Its perfectly fine and comfortable. I also don't understand the issues of using the touch screen. Usually after playing a game for one hour or two, you memorize the buttons or features on the gamepad and do not need to break the concentration to the TV.