Picked this up last week but didn't get to it until today. Just drained the battery of my iPhone 4S playing thorough most of the episode in one shot. In the lag-meter, the game is somewhere between Law and Order (no lag at all) and Jurassic Park (sound glitches, but okay performance). Generally I has no issues with performance, but I did use System Activity Monitor to clear out my RAM before starting. As far as gameplay is concerned, The Walking Dead is a huge step forward for TellTale, if only because choices matter. I find it interesting that making failing at a task (and getting yourself killed) feels much less consequential than saying the wrong thing or saving the "wrong" person. The level of interactivity is pretty much perfect for me, and even though you are clearly guided through the experience I didn't feel like I was just playing through the story but actually participating in it. The puzzles are not very difficult, but the moral choices you have to make can be excruciating. I have to confess that •••Spoiler: I chose to save Doug instead of Carly because she knew my secret and wasn't sure I could trust her. It was a crappy move and I kind of feel terrible about it (even though we're talking fiction and pixels here).
As someone who measures the quality of a game based on whether it reaches it and make me feel "something," I am thoroughly impressed. It says something that I'm not just looking forward to the next episode but actually want play again and see what changes if I behave differently. Can't speak about how it compares to the PC version, but I general I much prefer a touch interface with traditional adventure games, and the controls here feel well integrated and effective.
I'm sure this ground has been covered by many already, but I thought I'd add my two cents here.