Indie, exploration-based Terraria has developed quite a following since its release and now it's making its way to iOS courtesy of 505 games, which handled its recent console release. Ideally, 505 would have liked all three (PSN, XBLA, iOS) to have released simultaneously, but it turned out they would need more time to make sure to get it right; or, "to make sure it doesn't suck."
Taking a game designed with keyboard (and controller) in mind and making the iOS jump isn't the easiest of things. In fact, developments are still changing on a weekly basis. When I asked when the release date was, I was answered with a swift "when it's ready," which is a good answer to have (and to get). Still, what I played handled pretty well, and I particularly enjoyed that the time to do things like mine or chop down trees has been reduced.
I'm not sure how Terraria's core audience will feel about this reduction in time, but I loved it. The game is clearly being re-tooled with aware of the mobile mindset and I don't think the game suffers for allowing you to chop down trees a bit more quickly. A touchscreen joystick mapped to the bottom left handles locomotion. There is also an auto-jump for smaller ledges, though holding up on the joystick will elicit a higher jump should you want or need more control.
Interaction is handled either by taps and holds on screen, or with the aid of a sort of universal action button mapped on the bottom left, which will automatically activate things like tools and weapons to work on objects in your vicinity. While mining deep in the earth I found the action button a little lacking, as it kept leaving forehead-high blocks in my path that I'd then have to manually clear by selecting them on screen before I could progress -- but otherwise it handled well, though I didn't do much combat.
As a sidenote, Terraria is coming is Vita soon, too. Sony's portable might be better suited for the game, but I suppose if you don't have a Vita and need Terraria everywhere, then iOS could prove a decent option.