I recently purchased an intake for my 330ci. There are a few options available, but I chose the BMW Performance Intake. I purchased it from ECS Tuning for about $300 shipped.
The filter it uses is a normal panel filter stretched around a plastic frame. Currently, it is just a standard paper type filter, but I will replace it with an aFe high flow filter after I measure the size of it. The filter has more surface area than the stock filter box.
Installed in the engine bay:
Without a dyno, it is hard to gauge the intakes effectiveness. It does feel faster. There are dyno graphs on E46fanatics.com showing actual gains of aroun 7 hp (more than the 3 hp advertised), which is pretty good for a simple intake. I do not notice any difference in the sound of the engine, but this is most likely due to the overpowering tone of my exhaust. The intake itself is well made and compliments the engine bay very nicely. If this part was at the original MSRP, I would not buy it, but at $300 it is worth it. It’s a good foundation to start with.