My opinion, based on a few years on the highways and an hour or two on race tracks over the years:
Slotted rotors provide marginal improvements that aren't really important for 90% of the driving population. (I put them on my truck.)
Cross-drilled rotors are STRICTLY for the looks. They have no benefits on the street whatsoever and will marginally decrease braking performance. (This won't affect most people when you start with good brakes anyway, so if people want to put them on their vehicles for the looks, go ahead. Just as long as they understand it is for the look and not for any benefit.)
If you look at it logically, cross-drilled rotors don't make sense. There are no 'build-up of gases under the pad' issues with modern brake pads made in the last 20 years. They don't run cooler because there is no air flow out those holes. (They run sideways to the airflow and at 700 RPMs, there is no way any extra air can either escape or be taken in through the holes.)
In fact, if you think about it, they provide LESS braking power because they are lighter and are more prone to heating faster. Solid rotors or slotted rotors not only have more rotor surface for the pads to contact, they also have more metal and more mass.
Any rifle shooter will be glad to tell you there is a real good reason they INCREASE the mass and the surface area of a rifle barrel if it is needed to be consistently accurate shot-after-shot, even when warm. It is for this very reason that long-range accurate target rifles have much larger outer diameter barrels and the barrels are much heavier than your normal hunting rifle.
Simply transfer the analogy of the 'bull-barrel' target rifle that takes longer to heat up and can ultimately absorb more heat, to the brake rotor and one can see logically that drilling holes provides no benefits but is in fact marginally detrimental to performance.
By the way, some of the people who need to use their brakes the hardest (outside of race tracks) are police, fire and ambulance ... and they NEVER use cross-drilled brake rotors.
Personally, brakes are one place I don't worry about finding the cheapest parts on the market. I want the BEST parts on the market and this is why I would not buy anything except the premium line of rotors from well-known companies. But that's just me.
