Marble Math is a fun app for practicing basic math skills. The app is best when used as a companion rather than a primary app, as it doesn't explain math concepts and merely sharpens them.
Content-wise, the app is geared towards older juniors. The developers specifically mention that the app is designed for kids aged 9 to 12+, and judging from the problem sets, I agree with them. If you have younger juniors, I would suggest getting Marble Math Junior instead, which has the same gameplay but easier problem sets.