It was last available about this time last year, and Aldi is selling those Performance 29er Mountain Bikes again as "Special Buys" on 9 September. Fine reviews from FlowMountainBike.com from last year:
POSITIVES
- Ticks all the boxes necessary for light mountain biking.
- Quality Shimano gears and Tektro brakes.
- Neat frame.
NEGATIVES
- Sold in a box from a supermarket.
- Not for aggressive off road use.
- Medium and large size only.
Good beginners bike with Suntour XCM Fork with hydraulic lock-out, 18-speed drive train, internal cable routing & pannier rack mounting system. Aluminium frame, disc brakes, etc.
Update: Flow reviewed this year's model (thanks to MoneyPincher for the link). Didn't talk about the gearing difference though.
These are unbranded Polygon bikes which are sold at bicyclesonline.
Their cheapest model which might be the same as the Aldi one is $425.00
Very good specs considering the price.