Summary: Usually add these as front HD coolers and sidepanel graphics in Antec cases on the theory that more fans mean quieter operating fans. Guess I'm a traditionalist. Until Antec gives me a reason to go elsewhere, will continue to rely on their products.
Pros: Have used these in all my builds (5) for the last five years. Never had one go bad. Connected to a MOBO and using Bios or a control program (e.g., ASUS), they give you air when you need it and quiet when you don't. 120mm fans push a lot of air.
Cons: Pricey, but not when you divide the cost by 5 or 6 years of reliable cooling.
Summary: At 29dba, definately not silent, but not loud. Great for keeping your case cooler. The fan looks great and the 3pin/4pin connector is handy for installing when you might not have the right connector. Long life fan.
Summary: This fan can really move plenty of air, but since it's 120 mm, it doesnt have to operate at maximum speeds in order to move lots of air. This therefore makes it a quiet fan. You can also connect this to either your motherboard or to a 4 pin molex connector.
Summary: This case fan pushes a lot of air and gives excellent airflow throughout the case. The fan comes with both 4-pin and 3-pin connectors. Plugging the fan into my motherboard header allowed me to change the fan's speed. Running it at a lower speed was much quieter, without sacrificing too much airflow.