Motocross Challenge uses a virtual interface that resembles and plays like a GBA, but is tuned specifically for the game. The setup is one of the friendlier ones I've seen on the iPhone. I'm generally not a fan of virtual controllers. I understand the premise behind them, but most of the time controlling the game gets in the way of actually playing it. Buttons are placed along the edges of the iPhone's screen while gameplay takes place within a frame. It keeps your thumbs off the screen, but also reduces the amount of viewable space. Reducing an already smallish screen even further isn't ideal, but it's worth the tradeoff.
The actual buttons are incredibly responsive, probably a little too much. There were times where I accidently brushed the turbo button, sending my rider flying across the screen.
Motocross Challenge also makes use of the built-in tilt monitors for certain tricks. Though the concept makes sense, it was a bit problematic at times. Unlike other games that use the sensor, it's hard to tell when Motocross Challenge calibrates the initial setting. It's rare that you'll be able to hold the system in the ideal position all the time and it seemed like just the slightest shift in position was enough to throw the off the meter. As a result, my rider would go up on one wheel at odd times. A recalibration option was made available in a recent update, but it's a bit impractical and disrupts the flow of gameplay.
Motocross Challenge is one of the better games to hit the iPhone/ iPod Touch. As I noted in the GBA review a few years ago, what looks like a fast-paced arcade romp is actually more of a sim. Despite this, it is still incredibly accessible and easy to pick up and play, making it ideal for quick bursts of playtime.