It's kind of hard to tell with the Batman game due to the specific sounds, but how are the GamePad speakers? Are they better than the Wii Remote's?

Yes. Part of that is due to the actual data sent to the speaker (which is very low with the Wii Remote) versus the actual quality of the speaker.

I just started playing Trine on Steam this past week, and I'm impressed.  The footage of Trine 2 looks pretty great, and I think I'm going to hold out for purchasing the Wii U version if it has a lot of exclusive stuff.

Donkey Kong's Crash Course actually looks as if it would be frustrating.

Arkham City is still in my Steam backlog.  I started it some months ago, but the audio was still a little buggy in some of the areas.  I had thought to wait a bit to see if some patches or driver updates would help, but for some reason, I haven't felt compelled to go back to it.

I wouldn't call Donkey Kong frustrating, but it's definitely challenging. I never saw anyone actually complete the course, though I and a couple other guys got a lot closer than some people.