Drop in plastic bed liners offer tough materials that are all but impossible to damage. That makes for a strong alternative to more expensive spray in bed liners. Key to the toughness is the material in the typical drop in lining. You have to realize that you can dent a pickup bed, even with a plastic liner in the bed. But the plastic linings are a lot more likely to prevent that kind of damage.
Since spray on liners are really more of a paint, you get less dent prevention with the painted on liners as compared to what you get with a heavy plastic liner in place over your truck bed...
You should watch for just a couple of potential problems with these liners however. In some cases it will be better to skip the plastic models for another variety of bedliner. That's to avoid problems that may be hard to handle.
The latter two plastic bed liners are somewhat different. The DualLiner combines the attributes of the drop in bed liner with rubber truck bed mats.
Following are articles with more details and ideas.
Plastic bed liners work better for rough and tough loads.
Drop In Truck Bed Liners
More about drop in pickup truck bed liners?
Plastic Bed Liner
Plastic bed liners offer good value BUT watch out for the problems. They can cost you.
Drop In Pickup Bed Liner
The drop in liner beats a spray in liner in some ways...
Dualliner Truck Bed Liner
Combines plastic liners on sides with rubber bed mat on the floor...
Drop In Bed Liner
More on plastic bed liners for trucks...
Plastic Bed Liner
The plastic liner options...
Plastic Truck Bed Liners
Why drop in liners and why not...
Unlike with spray on bed liners, the plastic liners must be a custom fit for a particular truck model. That means, in many cases, your desired liner option may not work for your truck. That may force you toward the spray on option, as it will work on any shape or size of surface. But again, if you can get the DualLiner for your truck, it is worth a look.