Our 3+ year old Harbor Freight Tools 6x8 Greenhouse has at least one panel that needs to be replaced.

Cost Analysis:
Before we consider replacing the panel we'll have to decide if it is worth it to upkeep the greenhouse. See also:    • 3 Year Review of Harbor Freight Tools...  
We assessed the condition of the greenhouse and found that the most of the panels have at least a few more years of life. If we find that there are many panels to replace, it would be advantageous to replace the greenhouse with something more resilient (because the cost of the panels can add up quickly; or sneakily add up if a panels need to be replaced here and there.)

In our situation, the greenhouse is located where most of it is protected from the elements. There is one side that receives the brunt of our intense California sun and occasional wind storms. Since it is mostly one side, it seems worth it to maintain the greenhouse.

Panels:
There are many options for multiwall polycarbonate panels. The quality return on investment of the different brands (and thickness) is new territory for us. Our choice of panel will be a running trial to see how long they last and if it is worth the investment.

The panel is made by Palram.
It is 24x48 inch with a thickness of 5/16" or roughly 8mm.
We chose this panel based on Palram's reputation as a greenhouse manufacturer. We also wanted to get a more robust panel for the sun and wind exposed side of the greenhouse.
The 24x48 dimensions are much easier to handle and cut; say over larger sized sheets. This size is a good balance of price and convenience.

Additionally, it is important to note that we found that the 5/16" thickness is the maximum thickness for panels installed as roof panels.

Zone 10b / San Gabriel Valley / Los Angeles / Southern California / USA