Out of interest, I picked up one of these yesterday; only a few left, so must be popular.
As to the flatness, I'm no engineer, but placing a Veritas 36 inch straight edge at a number of points across the width, length and diagonals, I was unable to get a 0.05mm (.002 inch I think) feeler gauge under at any point, so I guess it must be flat enough.
I could have been lucky and got the good one, but at £7 it's surely worth a punt.