A drop in replacement panel air filter, oiled foam type. (it has three densities of foam, getting finer as the air passes through it.) Flows more air than standard (paper) type. It comes pre-oiled.
The Jamex filter was easy to fit. (as is any other replacement air filter for the Miata). The box claims you will get more power and a "sporty" sound. I detected a small but definite increase in power over the one year old stock filter, but could not detect any change in sound large enough to be sure it wasn't the brain playing tricks. The best part, is that the filter is cleanable (with a special cleaner and re-oiler pack- extra dollars.) It is also Guaranteed for life, so I wont be buying any more air-filters for this car.
If you think you are buying more power, you may be disappointed at the small increase, but if you think of it as a part that no longer needs to be replaced, then you've just got one or two horsepower for free.
I chose the Jamex, as it was easier to find around here than the K&N, I seriously doubt there is much difference between them, I just thought I'd point out that its not the only option.
I'd do it again...