Failures:
1. Battery short life, but you can fix it for like $30. Even fixed though is shorter than would like. Only 5-8 hours.
2. Cost - Wii U could be much cheaper without it and thus have more sales and maybe more 3rd party support

Things I like:
1. Some games use it great like Wind Waker, Zombie U, Assassins Creed, Batman Arkham City, Nintendo Land and a few others.
2. Playing when watching tv.

Whatever:
1. It's a controller, no more comfortable or less comfortable than any other controller imo. Since most games don't use the gamepad for anything but just a controller. It does its job as a controller. I don't feel the need to use another controller or anything. The gamepad is perfectly fine.

Overall:
Gonna wait to judge until I feel how Smash plays with it and Zelda U. As of now not enough games utilize it in order to justify the extra cost. Though I can't imagine playing Wind Waker or other games now without the gamepad.

It's been a massive disappointment so far. There is potential, but when even Nintendo are struggling to provide anything that delivers on this, at the moment I can't help but think they should look at doing a firmware update that removes the need for the pad and SKU without the pad and $100ish off the price of the console.

A resounding success for me. I keep on wishing my PS3 and 360 controllers had a touchscreen whenever I want to check a map and am forced to pause a game.

At this time, the gamepad has only 2 great features for me:
First ZombiU. Fantastic use of the gamepad, it's a damn shame that the first game I played on the Wii U has, by far, the best features. No other game even comes close.

Second, using it as a remote for my TV, Youtube and Netflix.

Other than that, the gamepad could be just an ordinary controller as far as I'm concerned.