Product review (5 stars): Just what we needed!
We had an older, "Rigid" duct.  Basically, it was thicker, single layer foil.  You bend it into a shape, and it stays.  So, when we pushed our dryer in, the old duct was crushed, reducing airflow.  It also tore everywhere it bent, leaving pinholes.

Neither a good thing, especially when using a gas dryer.

This duct is what we used for our replacement.  It's 6 layers of superfine foil laminated over a steel coil.  It stretches out to 8', but relaxes back to less.  It's still foil, so handle it less if you can.

This is the brand our appliance repairman recommends, and is pretty much "THE" UL duct.

Clean your ducts as often as necessary to maintain quick drying times.  If your dry times go up by 10%, then a little switch in your dryer is turning off hundreds of times for each load.  It wears out, and you either replace it yourself (an hour if you don't do it for a living, or $150-200 if you pay someone else).

You should use as LITTLE as possible.  Get the dryer as close to the wall as you can while still being able to reach both the back of the dryer and the wall.  Stretch out however much you need to go between the two, plus about 6 inches.  Use wire cutters to cut the steel wire, then scissors or a knife for the foil.  Work the clamps onto each end, about 3 coils in.  Work the end onto the wall, having it touch the wall, and move the clamp forward.  Do the same on the tiny nub hanging off of the back of your dryer.  Then push your dryer back into place.

We were able to use 3 of the 8 feet, so we have enough left that if it's damaged during the next duct cleaning, we'll just replace it.

You cannot use more than 8' of flexible duct to connect your dryer to the in-wall duct.  This is not the duct to use for in-wall or across-the house type stuff.  Ducts must be short to make sure you still have airflow.  Using too much causes more lint buildup, lower airflow (bad for gas dryer which gives CO2 and CO through the vent), and increases fire risk.  Also, if the airflow is too low, you can burn out one of the three safety sensors in the dryer.  Again, time or service call (parts are cheap if you can wait for them to be mailed).  If you bypass the safety sensors, then your house burns down and you end up on Mythbusters or the Darwin Awards.