Well, if you want a "hardcore" game that works really well (user-interaction wise) on the iPad, get "Baldur's Gate: Enhanced Edition" - a game originally released for the PC about 15 years ago and one of the best in it's genre (CRPG).
One word of warning though: this version is still quite buggy and might be considered an alpha release. Very enjoyable game though and I'm confident Beamdog will release updates in the coming months to further improve gameplay and reduce bugs.
Might also want to download the "12 days" app by Apple. Near the end of the month they will give some apps / music / books / etc... away for free. Last year got (among others) a "Broken Sword" (adventure) game this way.
+1 to Baldur's Gate. It's the right mix of nerdy humor, deep gameplay, and play-for-15-minutes you want in a thoughtful iOS game. It's also sold me on picking up Baldur's Gate II from Good Old Games once I'm done.
Three must have apps:
Goodreader: PDF reader which also plays audio and video, it's like having a proper file system on the iPad. I use this endlessly
Awesome note: great note taking app with todo features, link to calendar and great organizing features.
iThoughtsHD: mind mapping par excellence, and the author is really responsive to new ideas; he implemented a feature to automatically number sections so I could use the software for work breakdown structures
I just started using Textastic to code the next issue of our iPad magazine and I LOVE it. Freaking awesome app, can't say enough good things about it. On an iPad 1, never crashed, always smooth, its just a blast to use.
I also like Note Taker (forget if it's HD or Pro) for notes. It lets you zoom in on a part of the page and write there, which gives you a ton of control over how your page is laid out. Great app for going paperless.