Product review (4 stars): Perfect for me, with a couple of modifications...
First of all, let me start by sayin that this Heat Keeper might not be for everyone.  It all depends on your type of drier, your home's layout and your geographical location.
Please do yourself a favor and follow the safety instructions.  DO  NOT USE WITH A GAS DRYER!!!

We are currently using this in a small rental home with a crappy heater that dries the already dry New Mexico winter air.  It was a no-brainer to use the warm, moist air into the house to help heat it and increase the humidity to comfortable levels.

It is the first unit that we've ever purchased and I really don't regret it at all.
This unit is extremely flimsy and falling apart when you receive it.  However, it does not necessarily detract from its usefulness since, once installed, it is sturdy enough to not fall apart.  We haven't had any issues with it.
The vent in this house is already an exposed pipe going through the laundry room and is held up by brackets, I just made sure that this unit was next to one of the brackets and added a second one.  However, if it had it's own way of being held up, it would be much better.
Also, the two clamps that are included in the "kit" are a joke and should just be tossed.  Get yourself a couple of metal clamps that you can adjust better with a screwdriver and you'll thank yourself later.
The round lint trap is easy to pull on and off, an important feature due to the fact that the mesh is not fine enough to use as is and plenty of lint escapes into the house.
We actually like the filter mesh the way it is right now better because we only use it as a "filter holder."
- In the winter, we take off the round lint trap, put a used dryer sheet over the hole and hold it in place with the lint trap.  This holds it in place while in use.  Once the clothes are dry, we pull the lint covered dryer sheet off and replace it with the new one coming out of the dryer.  It is the easiest way to keep the lint trap clean, the dryer sheet catches the perfect amount of lint while letting the hot moist air in, and it only it takes seconds to replace with the next one, which the dryer supplies for you.
- In the summer, we take off the round lint trap, put a piece of a cardboard but to the size of the trap, and hold it in place with the lint trap.  This is absolutely necessary, otherwise, you will still get hot moist air coming into your home in the heat of the summer.  Yes, the flap is not perfect, but who cares, it is good enough for us.