Product review (5 stars): Works great for bathroom vent
This review is being written 1/1/2013 for the Deflecto TF4 4" Dryer Booster Fan advertised and sold by third party vendors on the Amazon website.

This unit can often be used to fix a broken or noisy bathroom vent van (no need to cut out the old unit and replace with a different-sized one).  First, disconnect the electricity and remove the existing fan from the housing.  Install the TF4 fan in the attic.  If you have a choice, install it farther away from the bath fan housing, and closer to the roof/wall exhaust vent.  Install wiring from the TF4 fan to the existing electrical connection in the bath fan housing (flexible conduit and wire nuts typically).  Connect the existing bath vent housing to the TF4 tube fan via flexible duct, and connect to the TF4 to the existing roof/wall vent via another piece of flexible duct.  Insulate the duct work to discourage condensation.  Reconnect the electricity and test.

The TF4 lasts for many years without the increased noise that I've experienced with the typical bath exhaust fan.  It's much quieter than the average unit, too.

You do need to have attic access to install this fan.

I'd recommend this fan due to its quiet and powerful operation, and long life.