Product review (5 stars): I'm happy that I did that
Despite of some negative reviews, I took my chance.  I'm happy that I did that.
Though the material may appear thin and flimsy comparing to the much thicker and harder vents sold in Lowe's or HomeDepot, this vent is just right for its job.  In fact, one could argue that this thinner and flimsier product is much harder to break than Lowe's/HomeDepot because of its flexibility.  I broke a few pieces that I bought from Lowe's and HD in the past.
I also really like the fact that its height is about half of the others which fit snugly underneath my TV console.  Its length is very sufficient to ensure the heat/AC will not be pushed / disappear into the furniture above it. I only need one piece and trimmed off 4" from its original length.  There is still another piece that can be easily taped together should the extension is under a much deeper couch.
I didn't bother use the included plastic clip because there is only one and if I ever clip it, I'll clip on all 3 sides.
Instead I use the 2-sided 1"-wide tape cut-to-measure so that all 3 sides of the EXTVENT are held firmly to the existing vent.
I could imagine this kind of installation method would help to divert the heat/AC upward from vents that are installed in the walls.