Cube aren't too bad generally. I bought my 8 year old son a Cube Acid 24 inch Mountain bike, nicely specced for the money, quite light and robust as well as being screwed together well. It's stood up well to all the abuse a small kid can throw at it.
The only grumble really is the paint finish, when new it looked really good for the money, but it scratches far too easily and where the cables touch the frame it rubbed through the paint really quickly, I should have put frame protection on straight away.
The bike you've been offered looks really well looked after in the picture above and sounds like a good price.