So how can I tell that
Marble Mixer was designed with the iPad in mind? The biggest clue is how multiplayer is handled. There is no online connection or some way to play with your friends on their own iPhones. Instead, each corner of the screen is designated to a player, and when you choose a human opponent, as opposed to a computer, a person has to physically stand there grabbing onto a corner of the iPhone. When you get three or four players circling the phone, you not only get a feeling of claustrophobia, but also the appearance of playing some technology-based version of a ouija board. Of all of the iPhone apps and games I've partaken of, this is the only one that I've seen attempt to handle multiplayer in this form, and the only way it makes sense is if the designers were planning it out on a bigger screen, i.e. an iPad.
In a related issue, the launching of your marble by flicking a finger out of your corner just doesn't seem to respond all that well. In a game where you are trying to land in a very precise location, typically as close to the hole as possible without going in, any control issues can make the game hard to bear. Once again, I feel like this is probably a cause of the game being played on a smaller screen. Without the kind of "touch resolution," for lack of a better phrase, that a system like the iPad has, it simply can't be expected to read how fast you are trying to flick your marble as well as you can on an iPad. Because of this, I found my marbles either shot off far faster than I wished, or fell flat and ended up not even making it to the target.
In the end, it's obvious the game wasn't designed with the iPhone in mind, and, while somewhat enjoyable, just doesn't work that well on the smaller screen. While I haven't had the opportunity to try it on an iPad, I can only imagine that the issues I've seen will lessen dramatically on the other iOS system. In short, pass up Marble Mixer on anything but the iPad.