[MadMax]

Well the stronger than steel promise is actually pretty vague. Steel varies considerably. High alloy aluminium that is forged (not machined) can acheive a strength comparable to unforged mild steel. Machined aluminium does not often get the same strength because it is cut into shape instead of being cold worked into shape. You actually have similar strengths to cast parts because the bulk stock is cast before it's machined to shape.

There are many HSS (high speed steel) cutters cut most alloy aluminums just fine. You don't usually need exotic cutters for aluminium. About the only case where I could see a benifit in superhard cutters is with cutting foamed aluminium. Aluminum can be blown into a very stiff, strong, air filled foam. Cutting it is a biatch because there is a lot of aluminum oxide which is nearly as hard as diamond. You wear out saw blades pretty fast cutting that stuff.

Machinability is also an important design concern for choosing a material. Materials which exhibit incredible mechanical properties but are very difficult to machine are pretty much limited to very high extreme/high value applications (like turbine blades) because making them into anything useful is nearly impossibly expensive.