If you search bimmer forums, they argue incessantly that the CAI will not increase HP, it is a farce. But, if you combine the intake with software, manifold, throttle bodies, exhaust, your engine will breathe better and you increase the HP. The CAI will give you a cool sound. I am fan of dinan because they research the hell out of their products. You are paying for a name, but sometimes you get what you pay for.
As for VF, they make great products, but they have not been in the BMW tuning scene nearly as long as Dinan. I know of a dinan super charger on an M3 that has 100,000 miles on it. I don't know of any VF supercharger reliable lasting 100,000 miles. Plus, Dinan will warranty their products on a brand new BMW for 4 years or 50000. That takes balls, but they stand behind their product.
If you are only planning to do a CAI, save your money and don't pay for the dinan, get VF or AFE.